protocol: Add release (destructor) request to wl_output

Outputs come and go, and this is needed to clean up wl_resources on the
server side. All protocol objects need a way to be destroyed.

Cc: Neil Roberts <neil@linux.intel.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
[Pekka: added commit message]