libweston: add heads_changed_signal

Add a hook for compositors to get a callback when heads are added or
their connection status changes, to which compositors likely want to
react to by enabling or disabling outputs (API for that to be added
later).

As many head changes as possible should be coalesced into a single
heads_changed call. Therefore the callback is made from an idle task.
This anticipates a future atomic output configuration API, where the
global output configuration is tested and set atomically instead of one
by one.

weston_pending_output_coldplug() needs to manually execute the
heads_changed call so that initial outputs are created before any
plugins get their start-up idle tasks ran. This is especially important
for ivi-shell which does not support output hotplug, and for tests to
guarantee the expected outputs.

v8:
- Change the callback function pointer into a wl_signal. The API is
  changed and renamed.

v6:
- fix a typo
- add comment in weston_pending_output_coldplug()

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
v6 Reviewed-by: Derek Foreman <derekf@osg.samsung.com>
v6 Reviewed-by: Ian Ray <ian.ray@ge.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Acked-by: Derek Foreman <derekf@osg.samsung.com>