libweston: add compositor list of heads

weston_compositor needs to maintain a list of all available heads, so
that a compositor can pick and choose which heads to take into or out of
use at arbitrary times. The heads may be on or off, and connected or
disconnected.

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>