wayland: Adds basic hybrid X + Wayland support

This adds support for running mutter as a hybrid X and Wayland
compositor. It runs a headless XWayland server for X applications
that presents wayland surfaces back to mutter which mutter can then
composite.

This aims to not break Mutter's existing support for the traditional X
compositing model which means a single build of Mutter can be
distributed supporting the traditional model and the new Wayland based
compositing model.

TODO: although building with --disable-wayland has at least been tested,
I still haven't actually verified that running as a traditional
compositor isn't broken currently.

Note: At this point no input is supported

Note: multiple authors have contributed to this patch:
Authored-by: Robert Bragg <robert@linux.intel.com>
Authored-by: Neil Roberts <neil@linux.intel.com>
Authored-by: Rico Tzschichholz.
Authored-by: Giovanni Campagna <gcampagna@src.gnome.org>

src/core/screen-private.h

@@ -257,4 +257,6 @@ void     meta_screen_workspace_switched (MetaScreen         *screen,
 
 void meta_screen_set_active_workspace_hint (MetaScreen *screen);
 
+Window   meta_screen_create_guard_window (Display *xdisplay, MetaScreen *screen);
+
 #endif