libweston: add weston_view_set_output()

Instead of desktop shell assigning view outputs directly,
use a new method, weston_view_set_output(). The method can
set up an output destroy listener to make sure that views
do not have stale output pointers.

Without this patch it is possible to end up in a scenario
where, e.g. configure_static_view() accesses memory that
has already been freed. The scenario can be provoked by
repeatedly plugging and unplugging a display. The faulty
memory accesses are reported by valgrind.

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>