Add Displayable._in_old_scene().

This is a method that can copy layers and screens when they're being
used as the old_widget in a transition.
Implement MultiBox._in_old_scene().
Optimize MultiBox._in_old_scene.

We now only use the newly-created boxes if at least one displayable
has changed. Otherwise, we don't bother, preferring to use the old
box (which likely has been cached).