to allow better re-use in GridfinityTrayLayout.
Mirror the base plate of the GridfinityTrayLayout so the engravings
for the feet are on "the bottom".
Related: #533
This is based on the TrayLayout2, but it designed to just give exactly
the correct size boxes to fit into a gridfinity system.
It has a modified TrayLayout setup, so it works in one step. It uses
javascript to auto-generate the grid, which means it doesn't currently
work for Inkscape or command line.