weston: migrate x11 to head-based output API

Migrate the x11 frontend to use the new head-based output configuration
API: listen for heads_changed, and process all heads.

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