FIELD OF THE INVENTION The present invention relates to a device for facilitating the bidirectional opening of leaves of doors, windows or similar structures. Generally, the leaves of doors, windows or similar structures (door-windows, awning windows, etc.) provide different opening modes. For example, the leaves can be hinged to the relative support structure, and in this case opening is allowed either clockwise or counter-clockwise, or both ways. PRIOR ART The opening of a leaf in the two ways, both clockwise and counter-clockwise, as described above is called a bidirectional opening. A number of devices for facilitating the bidirectional opening of leaves have been provided, such as hinges, pivots, leverages, etc Generally, the devices conventionally used to this purpose have a number of drawbacks. For example, they negatively affect the visual appearance of the structure to which they are applied. In fact, in most cases, these devices can be easily identified on said structure, since they project from one side of the leaf, or the relative support element or both. The need for minimizing the visual impact that these devices provide to the appearance (style, colours, design, etc.) of the structure with which they are associated, particularly in the door field, has been felt for a long time. Bidirectional doors and windows are usually provided with a traditional device in order to facilitate their opening, which is positioned on the upper portion of each leaf, a pivot being provided on the lower portion thereof. With this configuration, which is quite common, the leaf cannot be effectively rotated relative to its support structure. The object of the present invention is to provide a device for facilitating the bidirectional opening of leaves of doors, windows or similar structures, which overcomes the problems of the traditional devices, while being easy to mount. It is also an object of the present invention to provide a device for facilitating the bidirectional opening of leaves of doors, windows or similar structures, which does not affect the appearance of the leaf or structure being associated therewith, providing the designer of these structures with greater freedom of design. A further object of the present invention is to provide a device for facilitating the bidirectional opening of leaves of doors, windows or similar structures, which allows to set one or more stops for the leaf being associated therewith and adjust the force required to fasten/unfasten the leaf to/from said stops. SUMMARY OF THE INVENTION These and other objects are achieved by the present invention which concerns a device for facilitating the bidirectional opening, relative to a support element, of a leaf of doors, windows or similar structures, characterized in that it comprises: a housing provided with means for the integral coupling with said leaf, a lobed element being hinged to said housing and integral with said support element, said lobed element being co-axial with the opening axis of said leaf when coupled with said housing and said support element, - a tracer element being functionally associated with said lobed element, and slidingly fastened to a guide of said housing, thrust means for pressing said tracer element against said lobed element with a preset pressure. Preferably, the guide is transversal to the opening axis of the leaf, more preferably it is orthogonal thereto. When the leaf is assembled, the opening/ closure thereof defines a relative rotation between the tracer element and the lobed element. According to the present invention, the tracer element (i.e. the contour follower) can be provided with a wheel (i.e. a roller), at the end in contact with the lobed element, having the function of rolling along the perimeter of the lobed element all along the profile thereof. The thrust means can comprise an elastic element and an element for adjusting the pressure of said elastic element against the tracer element. According to a preferred embodiment of the invention, the elastic element is a spring housed within the above guide, being interposed between the adjusting element and the tracer element, and the adjusting element is a threaded pivot that can be meshed in a hole which opens in the guide in order to press/release the spring. Practically, the threaded pivot, together with the spring and the tracer element form a kinematic chain which slides within the guide of the inventive device, having the function of exerting a preset resistance against the relative rotation between the tracer element and the lobed element, which resistance depends on the preload of the spring, lobe profile, and relative position of the lobes being provided along the perimeter of the lobed element, relative to the tracer element. The housing of the device according to the invention can be fitted, all through the extension thereof, in a seat inside the leaf, which is preferably formed in the upper and/or lower crosspiece (or transom). The lobed element is integrally coupled to the support element of the leaf, such as the doorpost of the door frame. Consequently, said resistance against the relative rotation between the tracer element and the lobed element affects the opening/stop angles of the leaf relative to the doorpost. For this reason, according to the number and location of the stops that one desires to obtain relative to the opening of the leaf, the lobed element can have a variable number of lobes. Preferably, the lobes are at least two and more preferably they are four. According to an embodiment of the invention, the means provided for the integral coupling of the housing with the leaf comprise a plate that can be screwed to the leaf in the vertical position, for example on the leaf jamb, which can be associated with a spacer for adjusting the distance between the plate and the leaf. In practice, the spacer also allows to change the gap between the leaf and, for example, the doorpost being fastened thereto, such as to compensate any imprecision that may derive from the mounting of the leaf. The coupling between the lobed element and the support element of the leaf can be carried out in different ways, which are equivalent to each other. For example, the lobed element can be coupled with said support element by means of a bracket. A first portion of the bracket is preferably fixed into an end of the lobed element projecting from the housing and a second portion of the bracket is screwed to the doorpost. The device according to the invention can provide the coupling to means that have the function of preventing the leaf from opening according to a preset direction, such that the leaf can be opened preferably only clockwise or counter-clockwise. For example, the device of the invention can be functionally associated with a bracket that is integral with said doorpost, and provided with an end-of-stroke surface for the leaf. The device for facilitating the bidirectional opening of doors, windows or similar structures manufactured according to the present invention has a number of advantages over the traditional devices that are currently used for the bidirectional opening of leaves. The device according to the invention, substantially the whole volume thereof, can be housed in an inner housing of the leaf being associated therewith. Accordingly, the device is generally small-sized and its size can be adapted, upon manufacture, to the various sizes of the leaves with which it has to be associated. Due to this characteristic, the device being mounted, for example, on a door is hidden from view of the observers, which clearly brings positive effects on the visual appearance of the structure. This further allows the coupling of two devices on the same leaf, one on the upper crosspiece and one on the lower crosspiece, such as to obtain doors, windows, etc., each being fastened to the corresponding support structure, which have better functionality than the doors, windows, etc. fastened only at the upper crosspiece. By changing the preload of the spring by means of the threaded pivot, the inventive device allows both to effectively set one or more stops on the leaf opening/closure, and increase or decrease the opening angle within which the leaf is returned to close when it is released after it had been opened. For example, the device allows to set at least one stop between the closure point (corresponding to an angle of 0°) and the maximum opening point of the leaf in each direction (for example corresponding to an angle equal to ± 90°). This stop may be provided at an angle equal to ± 30°, ± 45°, etc. A further advantage of the inventive device over the traditional devices is that it allows to adjust the gap between the leaf and the support structure, for example the doorpost of the relative frame, by acting on the spacer in a very simple and quick manner such as to avoid any interference that may occur between the leaf and the doorpost. BRIEF DESCRIPTION OF THE FIGURES Further aspects and the advantages of the present invention will be better understood from the description below, which is to be considered by way of non-limiting example with reference to the annexed figures, in which: Fig. 1 is an exploded view of a device for facilitating the opening and closing of doors according to the invention; - Fig. 2 shows a perspective view of the device from Fig. 1 ; Fig. 3 shows a top plan view of the device from Fig. 1 ; Fig. 4 is a sectional view of the device from Fig. 3 taken along the line A-A; Fig. 5 is an exploded view of a possible assembly of a device according to the invention; - Fig. 6 is an exploded view of a possible assembly of a device according to the invention; Fig. 7 is a perspective view of three possible arrangements of a device according to the invention; Fig. 8 is a perspective view of three possible assembly configurations of a device according to the invention; Fig. 9 is a perspective view of three possible assembly configurations of a device according to the invention; Fig. 10 is a perspective view of three possible arrangements of a device according to the invention; - Fig. 11 is a sectional view showing the operation of a door provided with a device according to the invention. DETAILED DESCRIPTION OF SOME PREFERRED EMBODIMENTS OF THE INVENTION Fig. 1 is an exploded view of a device 1 according to the present invention. In the example illustrated, the device 1 comprises a housing formed by the two halves 2 and 3 that can be matched to each other. The housing may have different shapes, and generally it may either be made as one body or consist of several parts. The material making up the housing can be also selected according to such requirements as mechanical strength, colour, cost of the housing. The portions 2 and 3 from Fig. 1 are made from die-cast aluminium and can be clamped to each other by means of screws (not shown). In the case described, the housing section is square and has a 20 mm long side, whereas the housing is about 120 mm long. The housing can be fitted, preferably the whole volume thereof, in a suitable seat inside the leaf being coupled thereto. For example, the seat can be a groove formed longitudinally in the upper/lower crosspiece of the leaf. Though reference will be made herein below to the case where the device 1 is applied to door leaves, it should be understood that the invention is not limited to this one case. In fact, the device 1 can be generally coupled with window leaves, furniture wings, etc. The housing is provided with a seat 20 and a guide 11. The seat 20 has the function of supporting a lobed element 4 in the vertical position, i.e. such that it is coaxial with the axis A. In the example illustrated, the seat 20 provides an aperture 21 on each portion 2 and 3 having the function of engaging the lobed element 4 such as to allow the relative rotation between the housing and the lobed element 4. Between the lobed element 4 and each aperture 21 there can be interposed a bush 22 having the function of reducing the friction resulting from said rotation, and/or centering the lobed element 4 relative to the aperture 21. The bushes 22 can be manufactured from various materials. Preferably, they are made from a material having a low friction coefficient, for example Teflon or ABS plastic. The lobed element 4 can provide a varying number of lobes 10. As will be explained below, the number and location of the lobes 10 along the outer perimeter of the lobed element 4 have an effect on the stops and the return angles of the leaf being associated with the device. Preferably, as illustrated in Fig. 1, four lobes 10 are arranged symmetrically relative to the axis of the lobed element 4. This configuration corresponds to opening angles of die leaf ranging between +90° and -90° relative to, for example, the doorpost being fastened to the leaf. Generally, the lobed element 4 is made integral with the support structure of the leaf being coupled with the device 1. For example, in the case of a door, said support structure is the door frame. In this case, the lobed element is made integral with the door doorpost which, in turn, is integral with the masonry. The bracket 15 (Fig. 1-4) has tie function of fastening the lobed element 4 to the door doorpost. In the guide 11 there are slidingly housed a tracer element 5 and thrust elements 6 and 7 for said tracer element 5 along die axis O of the guide 11. The axis O is transversal to die axis A, preferably orthogonal thereto. The task of die thrust means 6 and 7 is to press the tracer element 5 against the lobed element 4 with a preset pressure, and such that, upon die relative rotation between the housing of the device 1 and the lobed element 4, the tracer element 5 follows the outer profile of die lobed element 4. The tracer element 5 is preferably provided, at the end in contact with the lobed element, of a wheel 12. The thrust means preferably comprise a spring 6 and a threaded pivot 7 which can be meshed within the guide 11 (Fig. 3 and 4) in order to change the preload of the spring 6 and accordingly change die thrust applied by die latter on the tracer element 5. Fig. 5, 6 and 11 show a possible use of the device 1 to fasten the door 9 to die doorpost 8 such as to obtain the bidirectional opening of the door 9. The bidirectional door 9 shown in Fig. 5 is coupled to two devices 1 according to die invention. The first device 1 is fitted within the suitable groove 17 formed in me upper crosspiece 30 of die door 9, the second device 1 is fitted in a groove formed in the lower crosspiece 31. According to die invention, when the device 1 is coupled to the door 9, it does not project from the profile of door 9. In practice, the volume of the device 1 is enclosed within die crosspiece 30 or 31, and accordingly the device 1 does not change die initial appearance of door 9. The coupling between die device 1 and the door 9 can be provided by means of the plate 13 illustrated in Fig. 5 and 6. The plate 13 is screwed botii to an end of the device 1 and the jamb 32 of die door. The distance between die plate 13 and the jamb 32 can be changed by means of a spacer 14, preferably a threaded dowel. When die device 1 is coupled to the door 9, the axis A can be displaced relative to the door 9. This allows to adjust die position of die rotation axis of door 9 relative to die doorpost 8 in a very effective and simple manner. Due to this characteristic, one can compensate any imperfections relative to the mounting of the device 1 on door 9, or imperfections of the doorpost 8. With reference to Fig. 5 and 6, the operation of the device 1 provides that the bracket 15 is integrally coupled with the doorpost 8, for example by means of screws or other equivalent means. When the door 9 is rotated, a relative rotation is established between the lobed element 4, which is integrally fastened to the doorpost 8 by means of the bracket 15 and the tracer element 5. This rotation is partially controlled by the kinematic chain consisting of the thrust means 6, 7 and the tracer element 5. Practically, said kinematic chain counteracts the displacement of the wheel 12 of the tracer element 5 on the lobes 10 of the lobed element 4. This allows one to set one or more preferred stops of the door 9 relative to the doorpost 8. In fact, the lobes 10 can be provided on the element 4 in such a position and number that the door 9 has, for example, three preferred stops at +90°, 0° and -90°, respectively, where with 0° is meant the position of the door when closed, and with +90° and -90° the position of the door 9 when opened clockwise or counter-clockwise (Fig. 11). It will be understood by those skilled in the art that the device 1 may also provide different angles for the stops discussed above. For example, these angles can be +75°, 0° and -90° and in this case the position of the lobes 10 will be asymmetrical relative to the axis A. The lobed element 4 can be interchanged according to the various requirements, such that the device 1 can provide the stops according to the desired angles. The pressure applied by the tracer 5 on the lobed element 4 and, consequently, the force required to bring the door 9 towards one of said stops or rotate the same starting from one of these stops can be simply changed by increasing/ decreasing the preload of the spring 6 by means of the pivot 7. By being capable of adjusting said force, one can advantageously avoid that a small air stream may undesirably cause the door 9 to close. A further advantageous characteristic of the device 1 is that when the door 9 is in an intermediate position relative to two preferred stops, for example between the open position at +90° and the closed position (0°), it is effectively returned to the closed position. In fact, when the wheel 12 rolls on a lobe 10 upon opening the door 9, the instant equilibrium of the kinematism formed by the elements 12 and 10 is of the unstable type. On the other hand, this equilibrium is stabile when the wheel 12 is between two lobes 10, this position corresponding to a stop for the door 9. Consequently, the tracer element 5 tends to move between two lobes 10 thereby causing the door 9 to rotate and be returned to a stop, which in this particular case is that corresponding to the closure of door 9. The assembly of the door 9 being coupled to the devices 1 is very simple. In fact, it is sufficient to secure each plate 15 to the same doorpost 8, then turn the devices 1 such as illustrated in Fig. 5 and couple the same to the respective seats 17 of the door 9. The device 1 can be operatively associated with means suitable to prevent that the door 9 may open in either way. The opening of door 9 can be thus changed from bidirectional to unidirectional, i.e. according to only one direction. The solution illustrated in Fig. 7-10 provides the use of an L-shaped bracket 16 that is integrally coupled with the doorpost 81 opposite said doorpost 8 in the same frame of door 9, according to three configurations (a), (b) and (c). With particular reference to Fig. 8(a), when the bracket 16 is mounted in this position it allows the bidirectional opening of the door that may be coupled to the same doorpost 8 by means of the device 1. On the other hand, when the bracket 16 is in the position illustrated in Fig. 8(b), the clockwise rotation (arrow O) of door 9 is prevented by the projection 40 that acts as the abutment surface for the door 9. Similarly, Fig. 8(c) shows the bracket 16 being coupled to the doorpost 81 according to such a configuration preventing that the door 9 may open in the counter-clockwise direction (arrow A). The door 9 can be provided with a traditional lock, though it is preferably provided with a magnetic lock. In fact, the device 1 has proved particularly effective in facilitating the opening/ closing of doors when the same are provided with magnetic locks. This type of locks allows a quicker change between the open-close conditions than the traditional mechanical locks, this being particularly advantageous when using bidirectional doors. In fact, when the bidirectional door 9 does not provide a stop in the closed position (rotation at 0°), it may rotate by crossing this position without the lock having the time to click. On the contrary, magnetic locks are very effective when used in association with the device 1 according to the invention. Advantageously, the device 1 allows the reversible assembly of door 9. In practice, the device 1 can be either associated with the first doorpost 8, or the second doorpost 81 of the frame. For example, the door 9 can be assembled on the relative frame either with the lock facing the doorpost 81, or with the lock facing the doorpost 8, according to the customer's requirements. The assembly in one of both configurations simply requires that the bracket 15 be secured on the doorpost 8 or 81.