LEVELLING SYSTEM FOR A PIECE OF FURNITURE The present invention relates to a levelling system for a piece of furniture. Pieces of furniture are generally provided with feet which keep them raised from the floor, allowing not only rapid and easy access underneath but also the regulation of their height and planarity. Adjustable feet for furniture have been known for some time. According to a first embodiment, of the eco¬ nomical type, illustrated in figure 1, a foot 1 has a threaded sleeve 2 pressure-inserted in the shoulder 3 of a piece of furniture 4 equipped with a bottom 5. The foot 1 has a threaded stand 6 equipped with a nut 7 to allow it to be screwed into the sleeve 2 which is effected with a spanner 8. This solution makes regulation extremely difficult above all for feet fixed to the rear part of the piece of furniture, which are difficult to reach by the operator. Another known device for regulating the height of a piece of furniture is that shown in figure 2, the level¬ ling device 10 is fixed to the shoulder 3 of the piece of furniture 4 equipped with a bottom 5, by means of a spe¬ cific bracket 11. The device can be operated from above by means of a screwdriver 12 or similar tool passing through a hole 13 situated in the bottom 5 of the piece of furniture. For so-called "day" furniture or furniture of a cer¬ tain value, the presence of holes, even if hidden by a cover, represents an unacceptable aesthetic fault, and the use this system is consequently limited. . A general objective of the present invention is ■therefore to provide a levelling system for a piece of furniture which can be operated from the outside on sin- gle feet also arranged in distant positions, for a simple levelling regulation of the piece of furniture. A further objective of the present invention is to provide a levelling system for a piece of furniture, which does not require anti-aesthetic drilling operations of its bottom. The objectives of the present invention also com¬ prise that of providing a- levelling system for a piece of furniture which is both economical to produce and easy to operate. Yet another objective of the present invention is to provide a levelling system which can be adapted to any form of furniture. In view of the above objectives, according to the present invention, a levelling system for a piece of fur- niture has been conceived, having the characteristics specified in the enclosed claims. The structural and functional characteristics of the present invention and its advantages with respect to the known art will appear more evident from the following de- scription, referring to the enclosed drawings, which il¬ lustrate a levelling system for a piece of furniture pro¬ duced according to the innovative principles of the in¬ vention itself. In the drawings : - figures 1 and 2 are perspective views of two levelling devices according to the known art; - figure 3A is an exploded perspective view of a level¬ ling device of the system according to the present inven¬ tion; - figure 3B is a perspective view of the levelling device of figure 3A assembled; - figure 3C is a raised side view of the device of figure 3B; - figure 3D is a raised side view of the device of figure - figure 3E is a plan view of the device of figure 3C; figures 4 and 5 are perspective views of assembly phases of the system according to the invention; - figures 6A-6D are perspective views illustrating vari- ous levelling regulation modes of the system according to the invention; - figure 7 is a perspective view of the system according to the invention assembled; - figure 8 is a perspective view illustrating various possible positions of the system according to the inven¬ tion; - figures 9A and 9B illustrate in a perspective view, two different embodiments of the system according to the in¬ vention; - figures 1OA and 1OB illustrate in a perspective view, two assembly phases of an optional detail of the system according to the invention; - figures HA and HB illustrate in an exploded perspec¬ tive view, a levelling device of the system according to a different embodiment of the present invention; - figure HC is a perspective view of the levelling de¬ vice of figure HA assembled; - figure HD is a raised side view of the device of fig¬ ure HC; - figure HE is a raised front view of the device of fig- ure HC; - figure 12A illustrates in an exploded perspective view, a levelling device of the system according to a different embodiment of the present invention; - figure 12B is a perspective view of the levelling de¬ vice of figure 12A assembled; - figure 12C is a plan view of the device of figure 12B; - figure 13A is a raised side view of the device of fig¬ ure 12B; - figure 13B is a raised section of the device according to the line A-A of figure 13A; - figures 14A, 15A, 16A, 17A, 18A, 19A, 2OA, 21A illus¬ trate in a perspective view, respective modes for fixing levelling devices to the shoulder of a piece of furni- ture; - figures 14B, 15B, 16B, 17B, 18B, 19B, 2OB, 21B illus¬ trate in a side section the devices of the respective figures 14A, 15A, 16A, 17A, 18A, 19A, 2OA, 21A. With reference to figures 3A to 2IB, a levelling system for a piece of furniture 24 equipped with a shoul¬ der 23, and a bottom 25, comprises a pair of levelling devices in turn comprising a shaped supporting element 40 and an extendable foot 30. The extendable foot 30, in an embodiment of the present invention, is produced accord- ing to the principles of the leveller according to Euro- pean patent Nr. 0 733 322 filed by the same Applicant. This disclosure is therefore considered as being in¬ corporated in the present description. Each extendable foot 30 is housed inside a support- ing element 40 to be fixed to the shoulder 23 of the piece of furniture and is arranged for being operated from the side, i.e. in a perpendicular direction to the translation axis of the foot, by means of a unified side engagement 31, preferably of the female type, such as a notch, a cross imprint or embedded hexagon and suitable for being engaged with the tip of a screwdriver 26 or hexagonal spanner 26' or similar tool. The system according to the invention comprises at last a pair of levelling devices fixed to the shoulder 23 of the piece of furniture by means of the supporting ele¬ ment 40. In particular, with reference to figure 4, this il¬ lustrates a first front extendable foot 30 and relative front supporting element 40 situated close to the front edge of the shoulder of the piece of furniture and a sec¬ ond rear extendable foot 30' and relative second rear supporting element 40' situated in a substantially rear position close to the back edge of the shoulder, i.e. in a position which is difficult to reach from the front ac- cess area. Said feet 30 and 30' and supporting elements 40, 40', only differ in the installation position and pref¬ erably, for understandable reasons of production economy, they are of the same type and are also symmetrical so that they can be interchanged and fixed indifferently to the right or left shoulders of the furniture. The extendable foot comprises a base 32, for contact with the floor, which is translated by means of a screw- nut screw as described in European patent Nr. 0 733 322. The extendable foot is housed inside the supporting element which has, for the purpose, a cavity 44 corre¬ sponding to the body of the extendable foot . In an embodiment of the present invention, the sup- porting element is equipped with a rear wall 41 for con¬ tact with the shoulder of the piece of furniture, said wall being equipped, for example, with pins 42 extending therefrom and through which there are holes 43 for the passage of fixing screws 27. Correspondingly, with reference in particular to figure 4, the shoulder of the piece of furniture com¬ prises holes 28 and 28' respectively for said pins and screws. Coming back to figures 3A-3E, the supporting element 40 is equipped with a series of horizontal ribs 46, pref- erably three, and vertical ribs 45, preferably two. The upper horizontal ribbing is also equipped with a hole 49 for the access of a tool if the extendable foot is produced to be operated from above through the upper engagement 33. In both of the vertical ribs there is a passage hole 47, in correspondence with the side engagement 31 of the extendable foot, so as to allow access o"f the tool for the levelling regulation of both sides. It is therefore possible to use the levelling device both in right and left shoulders by simply rotating the extendable foot by 180 degrees inside the supporting ele¬ ment . Furthermore, when these levelling systems have pins 42, they must be able to be assembled on the shoulder-of the furniture with a hammer rather than with a pneumatic pushing tool. The ribs 45 are designed so as to form, on the front part of the support, a kind of rest/pushing surface. On the outer edge of both of the vertical ribs 45, there are housings 48 suitable for receiving an extended element 80 engaging it so as to allow its rotation but not translation in a longitudinal direction. With reference to figures 5 to 9B, in order to also allow the levelling regulation of distant devices 40', 30', i.e. dislocated in a rear position, a pair of level¬ ling devices is fixed in alignment on the shoulder and the extended element 80 in the form of a cylindrical rod, is inserted between the two devices. Said extended element 80 has a male-shaped end 81 suitable for being inserted inside the engagement 31, and a female-shaped head 82, at the opposite end (or vice versa) , suitable for receiving a unified tool such as a screwdriver 26 with a cutting or star-shaped tip or a hexagonal male spanner 26' or similar tool. The element 80 can be produced in several modular parts and its ends can be of any type, obtained, for ex¬ ample, by combining different standardized forms such as a notch with a hexagonal male component, to mention two forms. Once the extended element 80 has been inserted in¬ side the coupling 31 of the rear extendable foot 30', it is hooked inside two housings 48 suitably shaped to allow rotation without translation and without becoming un¬ hooked unless a certain force is applied by the operator. In this respect, the extended element 80 is equipped with a smooth surface 83 and a restraint area 84 in the form of annular notching suitable for keeping the element hooked inside the housings 48 or, alternatively, a shoul¬ der spring 85 which prevents the translation of the ex- tended element 80 and possible unhooking from the cou- piing 31. As the pieces of furniture can have different sizes, and, due to assembly errors, the levelling devices can be positioned at different distances than those envisaged, the notched area 84 is extended for a suitable length to adapt itself to a wide interaxis range between the two supporting elements. Figures 9A and 9B illustrate with the curved arrows R the rotation possibility of the extended element, whereas the straight arrows T indicate the impossibility of translation in both directions . In an embodiment illustrated in figure 6D, it is also possible to operate from above on a device produced with a supporting element 60 equipped with an upper hole. In this case, there is a pass-through hole 29 in the bot¬ tom 25 of the piece of furniture, in which the tool 26 is inserted. With reference to figures 1OA and 1OB, in order to increase the resistance of the foot, thus improving the load distribution, it is possible to insert, between the levelling device and the bottom of the piece of furni¬ ture, a cylindrical body 70, equipped with an annular projection 71 at one end, suitable for engagement with the hole 49 situated on the upper horizontal ribbing of the supporting element 40. With reference to figures 11A-11E, these illustrate a levelling device which comprises supporting elements 60, 60' produced by the joining of two half-shells 61 and 62 shaped for directly housing the mechanism of the ex- tendable foot 30, 30' . In this embodiment, the extendable foot is housed inside two half-shells 65 situated in the supporting ele¬ ments 60, 60' . The extendable foot mechanism therefore envisages a screw 37 which holds the base 32 and is housed inside a hollow body 34 which contains a nut screw 38, the screw 37 rotates as a result of coupling with conical notchings obtained by means of a toothed head 35 coupled with a toothed crown 36 which is equipped with the side coupling 31 in order to be operated by the screwdriver 26 or hex¬ agonal spanner 26' . The two half-shells 61 and 62 are coupled and assem¬ bled thanks to the insertion of pins 63 into holes 64 situated in the half-shells themselves. In a different embodiment illustrated in figures 12A-12C, the supporting element 60, 60' also has an upper hole 67 so that the tool 26, 26' can be inserted from above to be engaged in an upper housing 39 situated in the head 35. This allows regulation also from above for systems requiring this. With reference to figures 13A and 13B, the support¬ ing element 60, 60', can be optionally equipped with chamfers 72 along the edges to avoid operating on sharp edges. With reference to figures 14A to 2IA and the corre¬ sponding sections illustrated in figures 14B to 21B, these illustrate respective embodiments of the fixing of the supporting element 60 (and obviously also 60') to the shoulder of the piece of furniture; these systems can naturally be used for any supporting element . The figures respectively illustrate: in figures 14A and 14B fixing with two pins 42 overlying two screws 27; in figures 15A and 15B, four notched pins 42 extending from the rear wall; in figures 16A and 16B in the rear wall of the supporting element, there are four holes for four screws 27; in figures 17A and 17B, a plate 68 extends from the base edge of the rear wall, for supporting the shoulder and the wall, has two holes for the screws 27; in figures 18A and 18B, the supporting element has the plate 68 and two pins 42 protruding from the rear wall; in figures 19A and 19B, the supporting element is as above with an extra two holes for the screws 27; in figures 2OA and 2OB, the supporting element has pins 42 protruding from the plate 68; finally, figures 21A and 21B illustrate a supporting ele¬ ment equipped with teeth 69 protruding from the end of the plate 68 for gripping the shoulder from below. From the above description, it is evident how it is possible to effect levelling, with a very simple tool, by acting on the front part of the piece of furniture. Furthermore, with reference to the figures, it ap- pears evident how a levelling system for a piece of fur¬ niture according to the invention is particularly useful and advantageous. The objective specified in the preamble of the de¬ scription is thus achieved. The forms of the levelling system according to the invention can naturally differ from those shown for purely illustrative and non-limiting purposes in the drawings. The protection scope of the invention is therefore delimited by the enclosed claims.