The present invention refers to an improved door, particularly for motor vehicles. In the field of manufacturing car bodies, and more generally bodies for any motor vehicle, conception and assembling of doors are important aspects since they raise problems regarding stiffness, water-tightness, aesthetics and have several mechanical, electro¬ mechanical and acoustic constraints. A door of a motor vehicle is currently composed, in general, of a metallic rigid structure which is the door backbone and comprising the framing of the glass part (window) and the external wall. On this structure, it is customary to access through its internal open part in order to install firstly the lock mechanisms, the window and its control mechanisms, possibly one or more loudspeakers, and a water-tightening sheet which avoids water and dust seepages inside the passenger compartment. In order to increase easiness of assembling and time saving on the assembling chain, when installing the mechanical members inside the door, so-called "cassette" systems are more and more used, in which the different members are pre-assembled in a single unit which is installed inside the door with a single assembly operation. The positioning and securing of this "cassette" are however rather difficult and uncomfortable operations, due to narrow available passages for such operations in the metallic door structure. The door assembling finally ends with securing the internal upholstery. This latter one is at the same time an aesthetic element and an acoustic insulating element, and does not contribute to the global door stiffness. Securing this upholstery, usually simultaneous with securing the handle, generally requires a certain number of screws and clips made of plastic material. Object of the present invention is solving the above prior art problems by providing an improved door, particularly for motor vehicles, which allows a better easiness of assembling and maintenance with respect to existing doors. Moreover, another object of the present invention is providing an improved door whose rigid carrier structure is composed of the part which, in existing doors, usually composes the internal upholstery on the passenger compartment side. A further object of the present invention is providing an improved door whose anchoring points to the body (hinges, lock) are integral with the rigid structure composing the internal upholstery of the door itself. A further object of the present invention is providing an improved door whose rigid structure composing the internal door upholstery also comprises, if necessary, the window framing. Moreover, an object of the present invention is providing an improved door in which the assembling of internal electro-mechanical members is performed by accessing through the external face of the door itself. Moreover, another object of the present invention is providing an improved door whose external face is composed of a removable panel which is able to be disassembled without the need of separating the door from the body. The above and other objects and advantages of the invention, as will appear from the following description, are obtained with an improved door as claimed in claim 1. Preferred embodiments and non- trivial variations of the present invention are the subject matter of the dependent claims. The present invention will be better described by some preferred embodiments thereof, provided as a non- limiting example, with reference to the single enclosed drawing, in which FIG. 1 shows an exploded view of an embodiment of the improved door according to the present invention. With reference to FIG. 1, it is possible to note an embodiment of the improved door according to the present invention; in this example embodiment, the improved door has been shown as a front left door of a car (not shown) . Such door comprises a main rigid internal panel 32 which corresponds to the internal upholstery facing the compartment interior, a "cassette" type structure 4 which joins a window 8, an electro-mechanical mechanism 10 for actuating the window and a loudspeaker 16, and an external upholstery panel 6 aimed to close the volume of the main internal panel 32. In FIG. 1 it is possible to see that the main internal panel 32 has a volume 20 which can be perfectly accessed for placing and securing the cassette 4. In fact, the stiffness of the internal panel 32 is obtained from the three-dimensional shapes and the different waving or' over-shims (created, for example, by members 24 and 16) which can be easily- modelled in the thickness of the panel made of plastic material or synthetic resin. It is therefore advantageous that in this way it is possible to have a volume available which can be perfectly accessed for installing and securing the set of electro-mechanical members contained in the door. It is further possible to add, in this volume, blocks of appropriate material adapted to limit noises, and/or absorb impacts in case of accident. If necessary, a bar 23 made of synthetic material or metal can also be secured in order to increase the resistance to side collisions (intrusion- preventing tube) . The installation of internal door members is completed by the securing of some accessories placed on the internal panel face towards the passenger compartment: internal door handle 21, armrests 24, electric and mechanical drives 10, 12, pockets 25 and other decorative parts such as listels made of precious wood. Assembling of a door according to the invention ends by installing the external panel 6 which can cover only the lower door part or, alternatively, cover at the same time the lower part and the framing/frame of the window 34. The improved door according to the present invention then comprises, as main and essential member, a main rigid internal panel 32 which is at the same time the door backbone and its internal face, commonly called internal upholstery^ This main internal panel comprises all the door face turned towards the passenger compartment, as well as the peripheral part which makes the door thickness. In case of cars of the saloon type, this panel comprises also the rigid framing 30 of the window. In case of bodies of the cabriolet type, the window framing does not exist and the main internal panel therefore only performs the functions of internal wall 30 of the passenger compartment and of peripheral band. In order to increase the comfort, aesthetics and functionality characteristics of the door according to the present invention, this main internal panel 32 is preferably made of composite plastic material, particularly plastic material reinforced by using fibres. For example, the main internal panel 32 can be moulded in polyester resin reinforced with woven or non-woven glass fibres, carbon fibres or polyamide fibres. The main internal panel 32 can also be realised, for example, through thermoforming of complex sheets of thermoformable plastic material or resin with reinforced fibres. Such main internal panel 32 can also be obtained by injection, using a plastic material loaded with short fibres. For example, composite materials marketed by company Saint Gobain can be used, such as, in particular, a thermoformable complex proposed with trademark Twin-tex®. The moulded or thermoformed main internal panel 32 is the backbone and the rigid structure of the door. In addition to shapes therefore linked to aesthetic and functional needs of an internal door face, in the main panel 32 of the door according to the present invention localised shapes and possibly over- shims 24, 16, 25 are provided, which are adapted to increase the door stiffness, both for its own support and for its own resistance to side impacts. In this way, the panel can have reinforcement outlines or over- shims which will be secured in the fastening points of the hinges 22, the lock 31 and the electro-magnetic block being part of the external handle 29 which supports, among others, the window 8 and its actuating mechanisms 10, 12, the loudspeaker 16 and the lock drive- 31. According to one of the preferred embodiments, the half-hinge integral with the door 22 is moulded with the main internal panel 32 in order to be a single piece. If necessary, the main panel can also be equipped inside it with a duct aimed to make conditioning air pass therein for rear vehicle places. Such duct, connecting the front edge and the rear edge of the door, can further and advantageously perform the function of protecting bar against a side impact. As previously mentioned, the main rigid panel further comprises the peripheral band of the door 33, namely the wall delimiting the door thickness included between internal door face and external door face. In an alternative variation of the present invention, it can be advantageous to provide, in the outline of this wall, one or a plurality of grooves or chases arranged for receiving flexible gaskets which ensure the water-tightness between door and body. The assembling of gaskets on the door according to the present invention, as replacement of gaskets commonly placed on the body, has the advantage of making these gaskets less exposed to impacts and rubbing generated by repeated passages of compartment passengers, which bring about, in time, the gasket wear and an unavoidable performance decrease. Obviously, the door according to the present invention further comprises all above-mentioned traditional electro-mechanical members, such as, in particular, the window 8 and. its driving mechanism 10, ' 12, the lock mechanism and drive 31 and, possibly, one or more loudspeakers 16; preferably, • and as already mentioned, the • set of these members is pre-assembled on a rigid frame, as a "cassette" 4, which is secured with a single operation onto the rigid structure composed of the previously -described main panel, even if, obviously, it is 'possible to provide different configurations of such members . The structure of the door according to the invention also implies a new assembling method of this cassette 4 which joins the electro-mechanical members: the assembling is performed on the external door side, being this latter one completely freed from structural constraints. This new assembling method is much easier than the traditional method, which consisted in introducing the internal door members through the internal face, practically always full of structure reinforcements made of padded sheet. This simplification has numerous and clear advantages both from the point of view of labour, deriving from the assembling time saving and the reduction of the risk of accidents, and from the point of view of assembling quality and reliability. These advantages are relevant also in case of possible repair of these electro¬ mechanical equipment placed in the door. The door assembling is completed by the laying of the external panel 6 which is secured by means of wings and screws onto the peripheral parts of the previously described main panel 32. This external panel 6 can be made of plastic material, for example of polyester resin reinforced with glass or carbon fibres, or of metal, for example of steel or aluminium plate. This external panel 6 in the door according to the present invention has only an ' essentially aesthetic role and only minimally contributes to reinforcing the door structure. It will be equipped only with gaskets, in particular the external window 35 gasket and possibly a peripheral and lock 27 gasket adapted to avoid noises and vibrations. Taking into account the structure of the main panel 32 which is the door wall on the compartment side, it is not necessary to embed into the door a water-tightening panel whose traditional function is preventing water seepages towards the passenger compartment. In fact, the main panel, being made of a rigid material, like reinforced plastic material, has an enough water-tightness as regards water and dusts. A good water-tightness is obtained by simply making an accurate coupling or by placing gaskets in the door areas aimed to receive the electric drives (window- lifting device 10, 12, rear-view mirror adjustment) or the mechanical drives (lock 31) . The innovative assembling concept deriving from the improved door allows installing the external panel 6 last, thereby limiting the risk of scoring and/or damaging the already-painted member. If the . window 30 frame must not be painted, or if it is coated by the external panel, the main panel 32 made of plastic material is the same one for all vehicles, whichever the colour is: in this way, it is possible to pre- assemble the door assembly, apart from the external panel 6, and differentiate at the last minute every door on the assembly line with a very simple operation which consists in securing the suitably painted external panel 6. Another advantage of the present invention results in case of a trivial accident or of simple scratches on the external panel: due to the easy disassembling and' the low cost of this piece, repairs can be quickly performed with much lower expenses than those deriving from traditional door repairs or replacements; in particular, when the hinges 22 are integral with the main internal panel 32, disassembling of the external panel 6 allows easily accessing the internal electro- mechanical members without having to employ the complete door disassembling. It is also worthwhile noting that the disassembling of the painted external panel 6 allows moving it away from the working area and thereby very highly reducing the risk of deterioration of the panel itself, and in particular of the paint, during maintenance or repair operations. For the same reason, door painting operations on the working line in cooperation with car ' body painting are highly simplified. Another advantage of the door according to the present invention is that it has a substantially reduced weight with respect to a traditional door whose structure is essentially made of steel: an easier and less fatiguing actuation follows for vehicle users, together with an easier handling and placement on the assembling line and a reduction of vehicle fuel consumption. Finally, the reduced number of door assembling operations allows advantageously using the existing robotic techniques, both for assembling the members composing the door and for its securing onto the car chassis.