This adds some type safety, since the super-layout is now of a distinct
type from the sublayouts. This avoids the ugliness we had, of passing
"fake" windows to the super layout. Now we directly lay out the layouts.
This is probably only ever useful in combination with Combo.
It sticks one window in a square region, and makes the rest
of the windows live with what's left (in a full-screen sense).
