TECHNICAL FIELD The present invention relates to a method and system for producing decorative paper and laminated panels incorporating such decorative paper, in particular decorative paper and panels for covering floor, furniture or wall surfaces. BACKGROUND ART Commonly used methods of producing decorative paper (subsequently laminated to produce floor panels or panels for covering various types of surfaces) substantially comprise impregnating a paper substrate (normally fed continuously off a reel) with a polymer resin; applying a mixture of polymer resin and abrasion-resistant particles (normally corundum) to the impregnated substrate (wet or dry) ; and, finally, applying a protective layer of polymer resin, preferably containing extremely hard, mildly abrasive particles (e.g. glass spheres) . In known methods, the various polymer layers, possibly containing particulate fillers, are coated on using various types of roller coating units. Such methods have various drawbacks. In particular, various roller coating units are required, each for applying a particular layer, i.e. a given polymer composition; the amount of resin applied is regulated by metering rollers on the roller coating units, which rarely provides for precise control, so that surplus resin is often applied (and must therefore be recovered and recirculated) ; and, unless complex, high-cost regulating systems are used, layer thickness regulation is normally also inaccurate. DISCLOSURE OF INVENTION It is an object of the present invention to provide a method and system for producing decorative paper and laminated panels incorporating such decorative paper, in particular decorative paper and panels for covering floor, furniture or wall surfaces, designed to eliminate the drawbacks of the known art. According to the present invention, there are provided a method and system as claimed respectively in the accompanying Claims 1 and 12. Using flathead coating devices according to the invention (i.e. coating devices, in which the coating material is dispensed through a substantially rectangular feed slit elongated crosswise to the substrate and bounded by a continuous peripheral edge) eliminates all the drawbacks typically associated with the use of conventional roller coating units. In particular, dispensing of the polymer composition and layer thickness can be controlled extremely accurately, by the polymer composition being dispensed through a slit having a precisely defined flow section, and by regulating the amount of polymer composition dispensed by simply and precisely controlling feed pressure. The same flathead coating device can also be used to apply layers of different thicknesses and widths by simply adjusting the size of the feed slit (in particular, by changing the spacer between the members of the head, as explained below) . The same flathead coating device can safely be used to apply different types of layers, i.e. different polymer compositions, even with various types of particulate fillers; and one flathead coating device may be provided with respective )feed slits to simultaneously apply two or more superimposed layers. BRIEF DESCRIPTION OF THE DRAWINGS A number of non-limiting embodiments of the invention will be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a schematic view of a system in accordance with the invention; Figure 2 shows a schematic exploded view of a coating head forming part of the Figure 1 system; Figure 3 shows a section of the Figure 2 coating head assembled; Figure 4 shows a section of a variation of the Figure 3 coating head; Figure 5 shows a schematic view of a further embodiment of the system according to the invention. BEST MODE FOR CARRYING OUT THE INVENTION Number 1 in Figure 1 indicates as a whole a system for producing decorative paper and laminated panels incorporating such decorative paper, in particular decorative paper and panels for covering floor, furniture or wall surfaces. System 1 comprises conveying means 2 (known and shown schematically by feed rollers 3, 4) for feeding a flexible (in particular, paper) substrate 5 in sheet or, preferably, continuous strip form through system 1; an impregnating unit 7; a first coating unit 8; and a second coating unit 9. In the example shown, impregnating unit 7, which is known, is located upstream from coating unit 8 in the travelling direction of substrate 5 through system 1, and comprises a tank 10 containing an impregnating bath 11; and means 12 for feeding substrate 5 into tank 10 and bath 11. Coating unit 8 comprises a flathead coating device 15 having at least one feed slit 16; and a guide unit 17 - defined, in the example shown, by two rollers 18, 19, between which substrate 5 is fed - for supporting a portion 20 of substrate 5 close to slit 16, with a face 21 of substrate 5 facing slit 16. The portion 20 of substrate 5 facing slit 16 winds about and rests on the outer surface of roller 19. Coating device 15 comprises a coating head 25 having slit 16 and fitted to an actuating system 26; slit 16 is connected by a conduit 27 to a tank 28 containing a polymer composition 29 to be applied; and a pump 30 pumps polymer composition 29 from tank 28 to slit 16. Actuating system 26 (known and shown only schematically) provides for moving head 25 with respect to substrate 5 and, in particular, for adjusting the distance between slit 16 and face 21, and possibly the tilt of head 25. With reference also to Figures 2 and 3, slit 16 is substantially rectangular, is elongated crosswise to the travelling direction of substrate 5, and is bounded by a continuous peripheral edge. Head 25 comprises two fixed facing members 31, 32 separated by a spacer 33. Members 3tL, 32 have respective substantially flat surfaces 34, 35 parallel to and facing each other; and respective lips 36, 37 fixed with respect to each other and defined by respective front edges of surfaces 34, 35. Surface 34 of member 31 has a groove 38 substantially parallel to lip 36 and connected to conduit 27 by a channel 39 formed through member 31. Spacer 33 is defined by a substantially C-shaped plate having opposite surfaces 41, 42 cooperating respectively with surfaces 34, 35, comprises a central recess 43 superimposed over a portion 44 of groove 38 and located between two sides 45 extending flush with lips 36, 37, and is gripped between members 31 and 32 by means, for example, of screws (not shown) inserted inside holes 46. The size of recess 43 and the thickness of spacer 33 determine the width and height of slit 16 and, therefore, the size of the layer of polymer composition 29 issuing from slit 16 and applied to substrate 5. The width and/or thickness of the applied layer can therefore be adjusted by changing spacer 33. With reference to Figure 1, coating unit 9 is located downstream from coating unit 8, and is a conventional known roller coating unit not described in detail for the sake of simplicity. It is understood, however, that coating unit 9 may also feature one or more flathead coating devices of the type described above. In the example shown, coating unit 9 comprises two coating assemblies 51, 52 located on opposite sides of <■ substrate 5 and defined by respective pairs of rollers 54, 55, between which respective polymer compositions 56, 57 are fed. System 1 implementing the method according to the invention operates as follows. Substrate 5 is fed into and impregnated in bath 11 of impregnating unit 7. Bath 11 is preferably an aqueous solution of one or more polymer components, in particular, melamine resin or similar and/or polyvinyl alcohol. The impregnated substrate 5 is then fed to coating unit 8. In the preferred embodiment referred to, the impregnated substrate 5 is fed wet to coating unit 8, with no drying steps in between. A layer 61 of polymer composition 29 is deposited through slit 16 onto face 21 of substrate 5, and preferably comprises at least one polymer component, in particular a melamine resin or similar or polyvinyl alcohol, and a particulate filler, in particular an abrasion-resistant substance such as corundum. Substrate 5 with layer 61 is fed from coating unit 8, possibly via a drier (not shown) , to coating unit 9; coating assembly 51 applies onto layer 61 a layer 62 of polymer composition 56 (preferably comprising at least one polymer component, e.g. melamine resin, and a mildly abrasive particulate filler, e.g. glass spheres); and coating assembly 52 applies a layer 63 of polymer composition 57 (comprising in particular, a polymer resin, e.g. melamine resin, with no particulate fillers) onto a face 64, opposite face 21, of substrate 5. An important aspect of the invention is that, to form two or more superimposed layers of respective like or different polymer compositions simultaneously, head 25 of coating device 15 has two or more superimposed slits for dispensing respective like or different polymer compositions. In the Figure 4 variation, for example, head 25 again comprises members 31, 32 described with reference to Figures 2 and 3, but, in this case, member 32, like member 31, also comprises a groove 38, and a channel 39 (connected to a tank of polymer composition) ; a substantially wedge-shaped auxiliary member 66 is interposed between members 31 and 32, and has two surfaces 67, 68 facing surfaces 34, 35 of members 31, 32 respectively; and two spacers 33a, 33b are interposed between member 31 and member 66 and between member 32 and member 66 respectively, to define two slits 16a, 16b. In actual use, one of slits 16a, 16b dispenses, for example, a polymer composition comprising at least one polymer component, in particular a melamine resin, and a particulate filler, e.g. corundum or other abrasion- resistant substance, and the other slit dispenses a polymer composition with no particulate fillers (e.g. a melamine resin) . In this case, layer 61 applied by coating device 15 is therefore defined by two superimposed layers!.applied simultaneously onto substrate 5 : a bottom first layer of polymer material containing the particulate filler, and a top second layer of polymer material only and superimposed on the first layer. In the Figure 5 variation, in which any details similar or identical to those already described are indicated using the same reference numbers, impregnating unit 7 is located downstream from coating unit 8 comprising flathead coating device 15. In this case, substrate 5 is first coated with layer 61 (comprising polymer resin and an abrasion-resistant substance, or comprising two superimposed layers : one bottom layer of polymer resin and abrasion-resistant substance, and a top layer of polymer resin only) , and is then impregnated.