well as a base for the same in accordance with a third embodiment of the present invention will be described below with reference to fig7 a and fig7 b referring to fig7 a according to the second embodiment of the present invention a shaft hole 13 39 is formed at the substantially central part of the base 1 39 between the projection 11 and the bifurcated portion 12 with this construction in contrast to the first embodiment of the present invention there is no need to form the slot 14 on the projection 11 the raising and lowering units 7 may be disposed either below the projection 11 or below the bifurcated portion 12 in the same manner as in the first embodiment of the present invention however each raising and lowering unit 7 is operated in a manner different from the first embodiment of the invention as described below it should be noted that when each raising and lowering unit 7 is provided with upper and lower limit stoppers there is no need to provide a spring corresponding to coil spring 6 in the first embodiment of the invention referring to fig7 b depicting a third embodiment of the present invention particularly in a case where it is.