EXT-1352 Add image art to floater backgrounds.

Implemented "legacy_header_height" hack to account for new art being 25 pixels
tall instead of legacy 18 pixels -- it auto-sizes the floaters to be taller.
Made all floaters have resize handles, just disable and make invisible if unused.
This simplifies the floater construction logic.  Floater header height now lives
in floater.xml, not as a global saved setting.  Reviewed with Richard.