Require all child objects to be destroyed before the parent. In other
words, all popups and surfaces created by one xdg_shell instance needs
to be destroyed before the xdg_shell object, otherwise a protocol error
is raised.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>