Emergency lowering device for a vehicle window The present invention concerns an emergency lowering device according to the preamble to claim 1. Vehicles, such as motor cars, lorries and buses, or marine vessels, are often fitted with opening side windows. Side windows are windows arranged in the doors or side panels of the vehicle. Certain side windows are often controlled using electric window lifters connected to the vehicle's electrical system. This means the windows cannot be opened if the vehicle's electrical system has been disabled in the event of an accident or fire, or if the vehicle is submersed in water. Consequently and especially if the vehicle is submersed in water, the occupants will not be able to exit the vehicle due to the pressure of water preventing the doors from being opened. Also in case of accidents where the doors or window frames have been damaged, preventing the windows from being opened, or if the vehicle is in such a position that the doors are blocked, the occupants must be able to exit through the windows. If the electrical system is disabled in the event of an accident or if the window frames are damaged so the window cannot be opened in a conventional manner, the occupants must break the window to be able to climb out. However, breaking the side window of a vehicle, for example, demands a good deal of force or some type of hammer, which is not always available in the vehicle. Previously known from JP 11022298 is an emergency lowering device for an electric side window in a vehicle. It comprises a cable extending to a catch arranged to the ordinary window lift. The window is raised and lowered respectively by a link arm system connected to a toothed cam and geared electric motor. In case of an emergency, for example if the vehicle's electrical system becomes disabled or if the window frame has been damaged and the motion of the window is inhibited, the ordinary window lift function can be disengaged with help of the cable. The cable acts on the catch, whereby the toothed cam will lose its interaction with the link arm system. The window will consequently fall through its own weight or can be pressed down by hand. The shortcoming of this arrangement is that if the window frame is damaged so that the window is wedged stuck, it will not fall through its own weight. Another shortcoming is that if the window is in its upper, closed end position when the window frame is damaged, a person attempting to exit the vehicle will not be able to press down the window by hand as the top edge of the window will be enclosed inside the window frame. The object of the present invention is to provide a device to enable a person occupying a vehicle to exit from the vehicle through a side window even when the door cannot be opened and the electrical system is disabled. This object is achieved by means of a device exhibiting the unique features and characteristics specified in the following claims. One embodiment chosen as an example will be described in the following with reference being made to the attached drawings, of which: fig. 1A shows a vehicle door without interior panel with the window in its upper, closed end position, fig. 1 B shows the vehicle door in figure 1A with the window lowered to its bottom, open end position using the ordinary window lift with a slack adjusting device in its active position, figure 2A shows the door from figure 1 B seen from the side along cross section X - X with the window in its upper, closed end position and figure 2B shows the door in figure 1 B seen from the side along cross section X - X with the window lowered to its bottom, open end position using an emergency opening device according to the invention. The emergency opening device shown in figures 1 and 2 are arranged in a vehicle door D, which in a well-known manner comprises a door leaf 2 with window frame 3. A window pane 4 is placed inside the window frame 3 and can be manoeuvred from an upper, closed end position 5 to a bottom, open end position 6, which is indicated with arrow A in figure 1A. It should be understood that the window can also be manoeuvred in the opposite direction. The window 4 slides in rails in the window frame 3, the window frame 3 often having a bowed shape to follow the rounded form of the vehicle. As a result, the glass of which the window 4 is made is bowed correspondingly. The window 4 can also be completely flat. In vehicles of so-called hard-top model, the doors do not have window frames, which instead comprises the top edge of the door through which the window slides and the section of the roof that adjoins the side panels of the vehicle. The adjoining roof section exhibits a strip into which the window slides when it is operated to its closed position. For operating the window 4 between its top 5 and bottom 6 end positions respectively, the window 4 is joined to a means of fastening comprising a support 7 equipped with a lift device 8 that is operated with a crank (not shown) or with an electric motor 9 connected to the vehicle's ordinary electrical system and operated with a pushbutton 10 or the like. In the window's 4 upper end position 5, the window aperture is completely closed by the window 4 and in its bottom end position 6, the window 4 is inside the door leaf 2, whereby the window aperture is fully or at least partially open. The lift device 8 can be designed in many different ways. For example, the support 7 arranged to the bottom edge 15 of the window 4 can be manoeuvred via a motorised gear wheel and a bowed toothed cam 11. When the motor 9 is driven clockwise, for example, the toothed cam 11 will turn anticlockwise, whereby the window 4 is lowered to its bottom end position 6. Another variant is a gear rack in the form of an encapsulated toothed cable that runs through a gear, whereby a support moves the window up or down depending on the direction of rotation of the gear. It should be understood that several different types of window lift can be used to manoeuvre the window. The emergency opening device 1 comprises a pulling device 12 fastened to the window 4 support 7, which is in the form of a rail or the like to which the window is screwed, glued or fastened in some other manner. The pulling device 12 comprises a control handle 13 with, for example, a cable, belt or chain 14 extending in the direction of travel of the window 4 when it is moved from its closed end position 5 towards its open end position 6. Essentially in the same position as the edge 15 of the window 4 is in when the window 4 reaches its fully open end position 6, there is a leverage device 16 through which the cable runs. The leverage device 16 comprises suitably two or three rollers 17 forming a construction similar to a block and tackle. Thanks to the leverage device 16 with its rollers 17, the window 4 can be forced to its lowered position 6 with a force that is greater than the force that is applied with the control handle 13. This is especially beneficial if the frame 3 in which the window 4 runs has been damaged or bent and is consequently obstructing the movement of the window 4 in the frame 3. Between the leverage device 16 and the window 4 support 7 according to figures 1A and 1 B, is arranged a slack adjuster 18. It comprises a spring 19 arranged between a point of connection 20 in the door leaf, for example, and a point of connection 21 in the cable 14, suitably in conjunction with the middle of the cable. The slack adjuster 18 is intended to take up any slack in the cable 14 that arises when the window 4 is moved to its open end position 6 with the conventional window lift 8 due to the cable 14 fastening point in the window's 4 support 7 following the window 4 on its way towards its open end position, while the cable's 4 fastening point on the leverage device 16 does not move. If the cable 14 is taut also when the window 4 is operated with the window lift 8, the cable may be dislocated from its interaction with the rollers 17 and prevent the emergency opening device from functioning. The spring 19 can comprise a torsion spring or a similar spring-action or elastic pulling device. The middle of the cable 14 is the position on the cable that is equidistant from the leverage device 16 and the window 4 means of fastening when the window is in its closed and raised position 5. The spring 19 can be fixed to the middle of the cable 14 but can well be arranged to slide with the spring being equipped with an eyelet, runner wheel or the like, through which the cable runs so that its point of connection can be displaced in a direction towards the support when the window approaches its open end position. As should be understood, the spring 19 point of connection 20 to the door leaf 2 is arranged as far as possible away from the cable, or at least as far away from the cable when the window 4 is in its raised and closed position 5 that the cable 14 is always in a taut state irrespective of where the window 4 is between its end positions 5, 6. It should be understood that the location of the slack adjuster 18 points of connection 20, 21 can vary depending on the design of the window 4 and window lift 8. The control handle 13 is located in conjunction with the leverage device 16, i.e. the bottom edge of the door leaf 2 that is adjoining the vehicle's sill 30. It comprises a grip piece 22 with which the user can grip the handle and preferably a brace 23 extending from one end of the grip section in a semicircle to the other end of the grip piece. The cable is connected to the mid-section of the semicircular brace. The control handle can also be designed with only a grip piece to which the cable is connected. When stored as shown in figure 2A, the handle is located in a depression 24 in the interior panel 25 of the door D, i.e. the side of the door D that is facing inwards into the compartment and which often exhibits an armrest 26 and a storage compartment 27. The depression 24 can be equipped with a hinged lid 28 which can well be in the same colour as the interior panel 25 for aesthetic reasons. The lid 28 hinge 29 may well be located so that the lid 28 is folded down towards the vehicle sill 30 when the control handle 13 is used. This means the lid 28 will be folded away from the area where the cable 14 becomes taut when the control handle 13 is used. The use of the device described above will be described in the following with reference made to figures 2A and 2B in the assumption that it is the driver who is exiting the vehicle through the window 4. It should be understood, however, that the method is the same irrespective of what window is being emergency opened. In the event of an emergency, for example if the vehicle is submerged in water and the vehicle's electrical system is disabled or in case of an accident where the doors become blocked and the window frames are damaged and obstructing the movement of the window in the frame, or if another emergency situation arises where the vehicle's passengers or driver must exit the vehicle through the window or the electric window lift is out of order, the driver is sitting in the driver's seat. The location of the lid 28 in the interior panel 25 is such that it can be reached by the driver's arm being lowered towards the vehicle's sill 30. The driver opens the lid 28 in the door panel 25, whereby the lid 28 is swung downward to reveal the control handle 13. The grip piece 22 is gripped and the handle 13 pulled upward towards the driver. Thanks to the location of the handle 13, the driver can apply considerable pulling power, partly by pulling with the arm and partly by leaning away from the door D. The increase in power through the leverage device 16 that is conveyed from the driver to the support 7 means that the resistance from the window lift 8 electric motor 9 and the resistance arising from the window 4 being stuck in a damaged window frame 3 can be overcome. When this resistance has been overcome, the window 4 support 7 is forced down towards the vehicle sill 30 taking the window 4 with it. The driver pulls the handle 13 away from the interior panel 25 of the door 1 until the window 4 opens enough for the driver to be able to crawl out through the lowered open window and can thereby securely exit the vehicle even if the door cannot be opened or the window cannot be opened in a conventional manner. The device has been described above based on the window being opened vertically from above and downward. It should be understood that the device can also be used on a window that opens horizontally, i.e. the window slides in a window frame from right to left or vice versa. It should also be understood that the device can be used with a window lift that is operated manually with a crank handle. If the window frame is damaged, the force that is applied with a crank handle to open the window can be too great and break the handle, which is why the user is in need of an emergency opening device as described above. The present invention is not limited to the above description and as illustrated in the drawings but can be changed and modified in a number of different ways within the framework of the idea of invention specified in the following claims.