TECHNICAL FIELD The present invention relates to a movable-jaw crusher for rubble and similar. BACKGROUND ART As is known, movable-jaw crushers are designed to crush rubble, rock, quarry materials, building and road demolition waste and similar, using the mechanical action of two jaws, which are positioned facing each other to define the two sloping lateral walls of a downward- converging hopper, and are moved cyclically towards each other to crush the material trapped between them, i.e. inside the hopper. More specifically, movable-jaw crushers normally comprise a casing or outer machine body, in which is formed a crushing compartment or chamber bounded laterally by four lateral walls, two of which are substantially vertical and extend parallel to and facing each other; two crushing jaws housed facing each other inside the crushing chamber to define the two sloping lateral walls of a downward-converging hopper; and a jaw actuating device for moving the two jaws cyclically against each other to crush the material trapped inside the hopper. More specifically, the two vertical lateral walls laterally defining the crushing chamber of the crusher define the two opposite vertical walls of the downward- converging hopper, while both the crushing jaws are defined by a flat metal front plate, or so-called "face plate", perpendicular to the two vertical lateral walls, and by a rear supporting structure which, depending on the type of jaw, is fixed rigidly or movably to the crusher casing. Most movable-jaw crushers, in fact, have a fixed jaw fixed rigidly to the crusher casing; and a movable jaw fixed movably to the crusher casing by means of a number of connecting members enabling the jaw to oscillate freely about a horizontal reference axis, under the control of an eccentric shaft powered by an electric motor or similar, while at the same time permitting a small amount of vertical translation. Obviously, the face plate is the part of the crushing jaw that comes into direct contact with the work material, and is therefore fixed to the rear supporting structure so that it can be changed easily when worn out. Though highly efficient, currently used systems of securing the face plate to the corresponding rear supporting structure are fairly complicated, and invariably call for dismantling at least one part of the crusher casing to change the face plate, thus greatly increasing the maintenance cost of the crusher. DISCLOSURE OF INVENTION It is an object of the present invention to provide a movable-jaw crusher enabling faster, easier replacement of the jaw face plates. According to the present invention, there is provided a crusher for rubble and similar, comprising two facing crushing jaws, and jaw actuating means for moving said crushing jaws cyclically against each other to crush the material trapped between the jaws; at least a first crushing jaw of said two crushing jaws comprising a rear supporting structure, a flat front plate resting on the rear supporting structure, and locking means for securing said flat front plate firmly to the corresponding rear supporting structure; the crusher being characterized in that said rear supporting structure comprises a flat supporting structure, on which said flat front plate rests, and in that said locking means comprise at least two projecting appendixes projecting from the rear face of said flat front plate and sized to engage an equal number of through openings formed in the body of the flat supporting structure; and a bolt locking member, which is movable on the surface of the rear face of said flat supporting structure, at one of said through openings, and is designed to engage and exert mechanical gripping force on the facing projecting appendix to draw the flat front plate onto the flat supporting structure and simultaneously move the flat front plate on said flat supporting structure, while keeping the flat front plate firmly against the flat supporting structure, so that the other projecting appendix of the flat front plate comes to rest on the inner edge of the flat supporting structure defining the corresponding through opening. BRIEF DESCRIPTION OF THE DRAWINGS A non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a lateral section, with parts removed for clarity, of a movable-jaw crusher in accordance with the teachings of the present invention; Figure 2 shows a larger-scale front view, with parts removed for clarity, of the Figure 1 crusher; Figure 3 shows a section along line III-III of the Figure 2 crusher; Figure 4 shows a larger-scale detail of Figure 3, with a number of component parts in a different operating configuration; Figure 5 shows a larger-scale detail of Figure 2, with parts removed for clarity. BEST MODE FOR CARRYING OUT THE INVENTION Number 1 in Figures 1 and 2 indicates as a whole a movable-jaw crusher, which is particularly suitable for crushing rubble, rock, quarry materials, building and road demolition waste, and similar. Crusher 1 substantially comprises a casing or outer machine body 2, in which is formed a crushing compartment or chamber 3 having a top opening, through which the work material is fed, and a bottom opening, through which the crushed material comes out; two crushing jaws 4 and 5 positioned facing each other inside crushing chamber 3 to define the two sloping lateral walls of a downward- converging hopper; and a jaw actuating device 6 for moving the two crushing jaws 4 and 5 cyclically against each other to crush the material trapped between the jaws, i.e. inside the hopper. More specifically, crushing chamber 3 is defined laterally by four lateral walls, two of which hereinafter indicated 7 - are substantially vertical and extend parallel to and facing each other in the example shown; while each of the two crushing jaws 4 and 5 comprises a flat front plate 8 made of highly strong metal and perpendicular to the two vertical lateral walls 7, and a rear supporting structure 9 which, depending on the type of jaw, is fixed rigidly or movably to casing 2 of the crusher. Each flat front plate 8 obviously rests on a corresponding rear supporting structure 9. With reference to Figure 1, flat front plates 8 of the two crushing jaws 4 and 5 are therefore positioned facing each other to define the two sloping lateral walls of a downward-converging hopper, while the other two lateral walls of the hopper are defined by the two vertical lateral walls 7 laterally defining crushing chamber 3. As regards rear supporting structures 9 of the two crushing jaws 4 and 5, in the example shown, crushing jaw 5 has a rear supporting structure 9 fixed rigidly to casing 2 of the crusher, while crushing jaw 4 has a rear supporting structure 9 fixed movably to casing 2 of the crusher by actuating device 6, which therefore provides for moving crushing jaw 4 cyclically against crushing jaw 5 to crush the material trapped between the two crushing jaws 4 and 5. More specifically, with reference to Figure 1, in the example shown, the top end of rear supporting structure 9 of crushing jaw 4 is fitted, to rotate freely, to the central portion of a horizontal supporting shaft 10 coaxial with a predetermined longitudinal axis and rotating eccentrically about a horizontal axis A of rotation parallel to and non-coincident with said longitudinal axis; while the bottom end of rear supporting structure 9 of crushing jaw 4 is fixed to casing 2 of the crusher by a number of connecting members 11 enabling crushing jaw 4 to oscillate freely about a horizontal reference axis, under the control of supporting shaft 10, while at the same time permitting a small amount of vertical translation. With reference to Figure 1, each crushing jaw 4, 5 also comprises locking means for securing flat front plate 8 firmly to the corresponding rear supporting structure 9. With reference to Figures 1, 2 and 3, rear supporting structure 9 of crushing jaw 5 substantially comprises a substantially rectangular flat supporting plate 13, on which flat front plate 8 rests; and a number of rear reinforcing cross members 14, which extend horizontally from one side of crushing chamber 3 to the other, are side welded to the rear face of flat supporting plate 13, and are fixed rigidly at both axial ends, by welding or similar, to the two vertical lateral walls 7 laterally defining crushing chamber 3. Flat supporting plate 13 obviously extends perpendicularly to the two vertical lateral walls 7 of casing 2, and is inclined at a predetermined angle with respect to the vertical, so that flat front plate 8 of crushing jaw 5 is inclined by the same angle with respect to the vertical when positioned, in use, with its rear face resting on the front face of flat supporting plate 13. As regards the locking means, crushing jaw 5 comprises a hand-operated, movable locking device 15 for selectively securing flat front plate 8 of crushing jaw 5 firmly, but in easily removable manner, to flat supporting plate 13 of rear supporting structure 9. With reference to Figures 2 and 3, movable locking device 15 comprises at least two projecting locating appendixes or pins 16 projecting from the rear face of flat front plate 8 and sized to engage an equal number of through openings 17 formed in the body of flat supporting plate 13; and a bolt locking member 18, which is fitted movably to the surface of the rear face of flat supporting structure 13, at one of the two through openings 17, and is designed to engage and exert mechanical tightening force on the facing projecting appendix 16 to draw flat front plate 8 onto flat supporting structure 13 in a direction perpendicular to the two plates, and to simultaneously move flat front plate 8 on flat supporting structure 13, while holding the flat front plate against the flat supporting structure, i.e. in a direction parallel to the plane of the two plates, so that the other projecting appendix 16 of flat front plate 8 comes to rest on the inner edge of flat supporting structure 13 defining the corresponding through opening 17. In the example shown, flat supporting plate 13 of rear supporting structure 9 comprises two pairs of through slots or openings 17, each of which comprises two substantially rectangular through openings 17 formed in the body of flat supporting plate 13 and aligned vertically one over the other. The two pairs of through slots or openings 17 are located on opposite sides of the central vertical plane M of the plate, and are aligned horizontally so as to form the vertices of a rectangle with its long sides positioned horizontally. In this case, movable locking device 15 therefore comprises two pairs of projecting appendixes or pins 16 projecting from the rear face of flat front plate 8; and two bolt locking members 18, each of which acts on the projecting appendixes 16 of a respective pair of projecting appendixes or pins 16. The four projecting appendixes 16 are obviously appropriately spaced and shaped so that each engages a respective through opening 17 when flat front plate 8 of crushing jaw 5 is positioned, in use, resting on flat supporting plate 13 of rear supporting structure 9. More specifically, with reference to Figures 1, 2 and 3, each pair of projecting appendixes or pins 16 of flat front plate 8 comprises two projecting appendixes 16 projecting from the rear face of flat front plate 8 and maintained coplanar with the same reference plane (coincident with the Figure 3 plane) which, in turn, is perpendicular to the body of flat front plate 8 and is positioned vertically when flat front plate 8 of crushing jaw 5 is positioned, in use, resting on flat supporting plate 13 of rear supporting structure 9. For the sake of simplicity, reference is made in the following description to a simplified configuration, in which movable locking device 15 comprises one locking member 18 and one pair of projecting appendixes or pins 16 with a respective pair of through slots or openings 17. With reference to Figures 3 and 4, bolt locking member 18 of movable locking device 15 comprises a lock wedge 19 resting on the rear face of flat supporting plate 13 to partly cover one of the two through openings 17 in flat supporting plate 13; and an inverted-U-shaped housing bracket 20 fixed to flat supporting plate 13, astride lock wedge 19, so that lock wedge 19 can only slide freely on the surface of flat supporting plate 13 in one predetermined direction dif which is preferably, though not necessarily, parallel to the straight line joining the two through openings 17 in flat supporting plate 13. Bolt locking member 18 also comprises a hand- operated cam actuating mechanism 21, which, on command, moves lock wedge 19, in direction dx, between a first operating position, in which the wedge maximizes the useful width of relative through opening 17 to permit insertion of the corresponding projecting appendix 16, and a second operating position, in which lock wedge 19 minimizes the useful width of the same through opening 17, and is positioned resting against the facing projecting appendix 16 to lock flat front plate 8 of crushing jaw 5 onto flat supporting plate 13 of rear supporting structure 9, as described above. Obviously, to ensure sufficiently stable connection of flat front plate 8 of crushing jaw 5 to flat supporting plate 13, the top of lock wedge 19 and the corresponding projecting appendix 16 are so shaped, at the contact area, that the mechanical forces produced by friction between the two bodies are so oriented to draw projecting appendix 16 inside through opening 17 and so draw flat front plate 8 of crushing jaw 5 against flat supporting plate 13. In the example shown, the two through openings 17 of movable locking device 15 are formed in the body of flat supporting plate 13 so as to be aligned vertically one over the other; lock wedge 19 is positioned resting on flat supporting plate 13 in the space between the two through openings 17; and the top of lock wedge 19 is shaped to comprise a flat contact surface 19a perpendicular to the central vertical plane M of flat supporting plate 13 and, at the same time, inclined with respect to the surface of the rear face of flat supporting plate 13 by an angle α of less than 90°. Similarly, the projecting appendix 16 of flat front plate 8, against which lock wedge 19 rests, is shaped to comprise, where it contacts the top of the wedge, a flat contact surface 16a perpendicular to the central vertical plane of flat front plate 8 (which plane is coincident with central vertical plane M when flat front plate 8 is fixed firmly to flat supporting plate 13) and, at the same time, inclined with respect to the surface of the rear face of flat front plate 8 by an acute angle preferably, though not necessarily, equal to angle α. Consequently, when lock wedge 19 is positioned resting against corresponding projecting appendix 16, so that flat contact surface 19a mates with flat contact surface 16a, flat front plate 8 is drawn onto flat supporting plate 13 in a direction perpendicular to the two plates, and, at the same time, is translated with respect to flat supporting plate 13 in a direction d2, parallel to the plane of the two plates, so that the other projecting appendix 16 of flat front plate 8 comes to rest on the inner edge of flat supporting plate 13 defining the corresponding through opening 17. Given the particular shape of the two flat contact surfaces 16a and 19a, direction d2 of flat front plate 8 is obviously also parallel to direction di of lock wedge 19 and to central vertical plane M of flat supporting plate 13. With reference to Figure 3, in the example shown, for a more stable connection of flat front plate 8 of crushing jaw 5 to flat supporting plate 13, the projecting appendix 16 which comes to rest on the inner edge of flat supporting plate 13 defining the corresponding through opening 17, i.e. the second projecting appendix 16, is also shaped so that the mechanical forces produced by friction between the edge of flat supporting plate 13 and the side of projecting appendix 16 are such as to further draw projecting appendix 16 inside through opening 17, and so draw flat front plate 8 of crushing jaw 5 against flat supporting plate 13. In the example shown, the projecting appendix 16 of flat front plate 8, which comes to rest on the inner edge of flat supporting plate 13 defining the corresponding through opening 17, is shaped to comprise, at the contact region, a flat contact surface 16b perpendicular to the central vertical plane of flat front plate 8 (which plane is coincident with central vertical plane M when flat front plate 8 is fixed firmly to flat supporting plate 13) and, at the same time, inclined with respect to the surface of the rear face of flat front plate 8 by an acute angle β preferably, though not necessarily, equal to angle α. With reference to Figures 4 and 5, cam actuating mechanism 21 comprises a cam-shaped cylindrical pin 22, which extends through the body of lock wedge 19, coaxially with a reference axis R substantially perpendicular to flat supporting plate 13, and is sized to project inside the through opening 17 underneath, with a portion of its curved lateral surface resting on a portion of the inner edge of flat supporting plate 13 defining the same through opening 17. Cylindrical pin 22 rotates freely about axis R inside the body of lock wedge 19, and is shaped at the end so as to rests on the inner edge of flat supporting plate 13 defining through opening 17 with a variable-profile annular portion 22a, a predetermined curved portion of which is spaced from axis R by a distance varying gradually as a function of the angle, while always being less than the nominal radius of cylindrical pin 22. Obviously, the difference between the nominal radius of cylindrical pin 22 and the minimum distance of variable-profile annular portion 22a from axis R corresponds to the travel of lock wedge 19. Given the above design, and the fact that cylindrical pin 22 remains, by force of gravity, resting at all times on the inner edge of flat supporting plate 13 defining through opening 17, cylindrical pin 22 and lock wedge 19 connected to it can be moved between the first and second operating position by rotating cylindrical pin about axis R. With reference to Figures 4 and 5, cam actuating mechanism 21 also comprises a transverse connecting rod 23 fixed rigidly at one end to the body of cylindrical pin 22, and extending perpendicularly to axis R, i.e. parallel to the rear face of flat supporting plate 13, so as to rotate about axis R together with cylindrical pin 22; and a screw or threaded bar 24 fitted through one of the two vertical lateral walls 7 of casing 2, and having an intermediate portion screwed to the distal end of transverse connecting rod 23 by a known mechanical articulated joint 25. In the example shown, the intermediate portion of screw or threaded bar 24 is screwed inside a cylindrical or spherical body, which in turn is inserted, to rotate freely, inside the distal end of transverse connecting rod 23. The head of screw or threaded bar 24 is therefore located outside casing 2, or rather outside crushing chamber 3, and is shaped to rest on vertical lateral wall 7 of casing 2 and to allow screw or threaded bar 24 to be tilted slightly with respect to the substantially horizontal reference position. In the example shown, cam actuating mechanism 21 comprises a cup-shaped body 26, which has tensioning springs inside, is fitted to the portion of screw or threaded bar 24 projecting outside casing 2, or rather outside crushing chamber 3, allows screw or threaded bar 24 to rotate freely about its longitudinal axis, and has a curved bottom resting on the adjacent vertical lateral wall 7 to allow screw or threaded bar 24 to oscillate slightly with respect to the substantially horizontal reference position. Operation of movable-jaw crusher 1 is easily deducible from the foregoing description with no further explanation required. As regards hand-operated, movable locking device 15 of crushing jaw 5, on the other hand, by acting on the head of screw or threaded bar 24 projecting outside the crusher, the operator is able to move connecting rod 23 directly, which, in turn, rotates cylindrical pin 22 about axis R to move lock wedge 19 from the first to the second operating position, or vice versa, and so quickly and safely lock/release flat front plate 8 of crushing jaw 5 to flat supporting plate 13 of rear supporting structure 9. Hand-operated, movable locking device 15 has numerous advantages, by enabling extremely fast replacement of flat front plate 8 of crushing jaw 5, with no need to dismantle any part of casing 2. Movable locking device 15 as described and illustrated above also has the advantage of comprising reasonably priced, commonly marketed, mechanically dependable component parts. Clearly, changes may be made to crusher 1, as described and illustrated herein, and to movable locking device 15 without, however, departing from the scope of the present invention.