Add compatibility with GTK+ 2.18

To replace all calls to deprecated code, GTK+ 2.20 is required - add
some basic compatibility code, so that it is still possible to build
mutter with GTK+ 2.18 when not using -DGSEAL_ENABLE.

https://bugzilla.gnome.org/show_bug.cgi?id=595496

src/tools/mutter-window-demo.c

@@ -24,6 +24,8 @@
 #include <X11/Xatom.h>
 #include <unistd.h>
 
+#include "gtk-compat.h"
+
 static GtkWidget* do_appwindow (void);
 
 static gboolean aspect_on;