the connecting member 15 is preferably a bolt where the head of the bolt prevents the tracks 1 2 from completely detaching from the frame members 5 6 but is not completely tightened to maintain the tracks 1 2 flush with the frame members 5 6 thereby allowing the tracks 1 2 to move in the plane perpendicular to the plane of the opening 4 while such positioning of the connecting member 15 allows the track edges 12 to separate from the frame members 5 6 upon the application of an external force it is preferable that in the absence of such a force the track edges 12 abut the frame members 5 6 thus as best shown in fig2 u2013 3 a compression spring 101 and a washer 102 are preferably located between the head of each connecting member 15 and its corresponding tracks 1 2 the tension of the compression springs 101 are preferably weak enough to permit the track edges 12 to separate from the frame members 5 6 upon the application to the apparatus 10 of an external force yet strong enough to urge the track edges 12 to return to their original positions abutting the frame members 5 6 upon the termination of such external force in an alternate embodiment shown in fig4 the present invention has a track.