Implements the xwayland_shell protocol which makes the surface
association happen via a shared serial, rather than sharing a wl_surface
resource ID across an X atom.

This solves a race that can happen if the wl_surface
associated with a WL_SURFACE_ID for a window was destroyed before the
update of the atom was processed by the compositor and another surface
(or other object) had taken its id due to recycling.

Closes: #1157

Signed-off-by: Joshua Ashton <joshua@froggi.es>
Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>