libweston: add weston_head destroy signal

Add support for subscribing to weston_head destruction.

The primary use case for heads being destroyed arbitrarily is the
DRM-backend with MST connectors, which may disappear on unplug. It is
not just the connector becoming disconnected, it is the connector
actually disappearing.

The compositor needs to know about disappearing heads so that it has a
chance to clean up "orphaned" outputs which do get disabled but still
need destroying at run time. Shutdown would destroy them as well.

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