Move weston source to compositor/

Move weston source to compositor/

This is the start of separating weston-the-compositor source files from
libweston source files.

This is moving all the files related to the 'weston' binary. Also the
CMS and systemd plugins are moved.

xwayland plugin is not moved, because it will be turned into a
libweston feature.

To avoid breaking the build, #includes for weston.h are fixed to use
compositor/weston.h. This serves as a reminder that such files may need
further attention: moving to the right directory, or maybe using the
proper -I flags instead.

v2: Move also screen-share.c, and add a note about weston-launch.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Tested-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Tested-by: Benoit Gschwind <gschwind@gnu-log.net>
Acked-by: Benoit Gschwind <gschwind@gnu-log.net>
[Pekka: rebased]