workspace: Smarten assert in light of O-R windows

O-R windows appear in workspace->windows, which aren't relocatable,
so we can't simply check if the workspace is empty after relocating
all normal windows, since those windows remain.

Make sure that the only windows we have are those that are
on_all_workspaces.
workspace: Don't try to move O-R windows when relocating workspaces

All we'll get for it is spam.