desktop-shell: detect stale shell surface outputs

When displays are hot (un)plugged, it may happen that
a shell surface is left with a stale pointer to an output
that has already been freed. Add an output destroy listener
to catch such situations and set the output pointer to NULL.

Signed-off-by: Semi Malinen <semi.malinen@ge.com>
Signed-off-by: Fabien Lahoudere <fabien.lahoudere@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>