Add MetaWindowActorX11 and MetaWindowActorWayland

Those are stub specialized classes for MetaWindowActor. This will
help ensuring that we do not execute X11-specific code paths on
pure Wayland clients.

The relationship between the window actor and the surface is the
following:

 * Wayland: MetaWindowActorWayland + MetaSurfaceActorWayland
 * X11: MetaWindowActorX11 + MetaSurfaceActorX11
 * Xwayland: MetaWindowActorX11 + MetaSurfaceActorWayland

It is not possible to have MetaWindowActorWayland backed by a
MetaSurfaceActorX11 surface.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/368

src/compositor/meta-window-actor-x11.c

@@ -0,0 +1,41 @@
+/*
+ * Copyright (C) 2018 Endless, Inc.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
+ * 02111-1307, USA.
+ *
+ * Written by:
+ *     Georges Basile Stavracas Neto <gbsneto@gnome.org>
+ */
+
+#include "compositor/meta-window-actor-x11.h"
+#include "meta/meta-window-actor.h"
+
+struct _MetaWindowActorX11
+{
+  MetaWindowActor parent;
+};
+
+G_DEFINE_TYPE (MetaWindowActorX11, meta_window_actor_x11, META_TYPE_WINDOW_ACTOR)
+
+static void
+meta_window_actor_x11_class_init (MetaWindowActorX11Class *klass)
+{
+}
+
+static void
+meta_window_actor_x11_init (MetaWindowActorX11 *self)
+{
+}