clx-interface: Use a mirrored SHEET to create our window.

  * This is a fairly big milestone, even if some of the design for
the clx-interface / rendering_1 split is starting to seem a poor
fit for a CLIM world.

  * We define a currently-minimal sheet class that can serve as a
child sheet, have it adopted by a graft, force it to be mirrored,
and then use the mirror as our window.  And it all WORKS.