xdg-shell/toplevel: Chain multiple parent-child relationships

Change the semantics of xdg_toplevel.set_parent to allow chaining
multiple parent-child relationships together, while allowing
arbitrarily unmapping parents, while keeping what is left over of the
chain intact.

This makes things easier to manage when parent-child relationships
cross client borders, for example when using xdg_foreign.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Signed-off-by: Mike Blumenkrantz <zmike@osg.samsung.com>