libweston: introduce weston_head

In order to support clone modes, libweston needs the concept of a head
that is separate from weston_output. While weston_output manages buffers
and the repaint state machine, weston_head will represent a single
monitor. In the future it will be possible to have a single
weston_output drive one or more weston_heads for a clone mode that
shares the framebuffers between all cloned heads.

All the fields that are obviously properties of the monitor are moved
from weston_output into weston_head.

As moving the fields requires one to touch all the backends for all the
assingments, introduce setter functions for them while we are here. The
setters are identical to the old assignments, for now.

As a temporary measure, weston_output embeds a single head. Also the
ugly casts in weston_head_set_monitor_strings() will be removed by a
follow-up patch.

Libweston major version is bumped, because weston_output struct layout
is changed.

v7:
- Bump libweston major version.

v6:
- adapt to upstream changes in weston_output_set_transform()

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
v5 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>