supporting segment 18 that is the raised stop button 80 rises above the plane of the second supporting segment 18 a second raised stop button 82 is located near the lower region 30 of the second supporting segment 18 the second raised stop button 82 projects upward from the obverse side 32 of the second supporting segment 18 the second raised stop button 82 is located approximately at a point one half of the width w of the second supporting segment 18 a first slidable frame piece 100 is shown in fig1 the slidable frame piece 100 is conveniently manufactured from a relatively rigid plastic such as polyvinyl chloride the first slidable frame piece 100 may be extruded and cut into appropriate lengths the length of the slidable frame piece should be slightly less than the width of the supporting segment 16 and accordingly the width of the supporting segment 18 as best seen in fig3 is a sectional view of the first slidable frame piece 100 taken along lines 3 u2014 3 of fig1 the first slidable frame piece 100 is generally t shaped and has a base 104 a leg 110 and a leg 112 as is shown in fig4 the base 104 at least partially defines a centrally located void 118 the centrally located void 118 extends along the length of the base of the first slidable frame piece 100 the leg 110 and the.