libweston: support user data on weston_output

Support attaching custom data to a weston_output by the traditional
destroy listener / wl_signal_get approach.

Needs a new destroy signal, because user data lifetime should be the
lifetime of the weston_output regradless of its enabled status. The old
destroy signal is for output consumers that only care about enabled
outputs in the system and gets emitted on disable, not on destroy.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Acked-by: Derek Foreman <derekf@osg.samsung.com>