libweston: add weston_head::connected

Heads may be disconnected or connected and the compositor needs to be
able to know the state to know which heads to take into use.

Currently a single head is automatically created with an output, and
outputs are only ever created as connected and destroyed on
disconnection, so it suffices to set connected to true. In the future,
backends are expected to create heads for both connected and
disconnected connectors, so that a connector can be forced on without it
being actually connected.

v6:
- split weston_head_is_enabled() to a new patch

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>