libweston: add weston_head_is_device_changed() API

Reacting to DRM hotplug events is racy. It is theoretically possible to
get hotplug events for a quick swap from one monitor to another and
process both only after the new monitor is connected. Hence it is
possible for display device information to change without going through
a disconnected state for the head.

To support such cases, add API to allow detecting it in the compositor.

v6:
- change str_null_neq() to str_null_eq()
- rename weston_head_condition_device_changed()
- move the condition from weston_head_set_device_changed() to the
callers

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>