Use weston_view_move_to_layer() consistently in desktop-shell

Most of this series is a pile of cleanup to make desktop-shell a bit more understandable. As a bonus, we start using weston_view_move_to_layer() everywhere, rather than manually juggling stuff around and guessing at what the end result might have been.

This is part of a yak-shave to unblock being able to enforce more regular constraints around surface/view mapping, as well as the API cleanup of unexporting manual layer moving.