libweston: move wl_output to weston_head

The wl_output protocol interface exposes things like monitor make,
model, sub-pixel layout and physical dimensions. Obviously wl_output is
meant to represent a monitor.

The abstraction of a monitor is weston_head. Therefore move the wl_output
global and the bound resources list into weston_head.

When clone mode gets implemented in the future, this means that monitors
driven by the same CRTC will still be represented as separate wl_output
globals. This allows to accurately represent the hardware.

Clone mode that used separate, not frame-locked, CRTCs to drive two
monitors as clones would necessarily also be exposed as separate
wl_output since they have different timings.

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>
Reviewed-by: Ian Ray <ian.ray@ge.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Acked-by: Derek Foreman <derekf@osg.samsung.com>