PLAN2D.3 Stacked floors in one model space

Asked: draw the first floor on top of the ground floor and turn each off, "stacking model spaces, not layers", without Xrefs. Goes further than SU.912's design layers (a layer with a scale and an elevation).

Built in the lab: FLOORS panel, FLOOR, one active floor; other floors show faded, snap but can't be edited; a viewport can show one floor; move or copy the selection to a floor; FLOORLAYERS for a file going to someone without floors. Layers run across floors. Saved in the file.

Not done: other programs show all floors together unless FLOORLAYERS is run. Floor elevation is stored only, nothing uses it yet. Stairs and anything that spans floors.