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]

README

@@ -114,6 +114,11 @@ Details:
 
 - xwayland ???
 
+- weston-launch is still with libweston even though it can only launch
+  Weston and nothing else. We would like to allow it to launch any compositor,
+  but since it gives by design root access to input devices and DRM, how can
+  we restrict it to intended programs?
+
 There are still many more details to be decided.
 
 
@@ -141,13 +146,13 @@ would be roughly like this:
 - fbdev-backend (depends on libudev...)
 
 - rdp-backend (depends on freerdp)
-	+ screen-share
 
 - weston (the executable, not parallel-installable):
 	+ desktop shell
 	+ ivi-shell
 	+ fullscreen shell
 	+ weston-info, weston-terminal, etc. we install by default
+	+ screen-share
 
 - weston demos (not parallel-installable)
 	+ weston-simple-* programs