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 is still possible to build the
shell with GTK+ 2.18 when not using -DGSEAL_ENABLE.

https://bugzilla.gnome.org/show_bug.cgi?id=618258

src/shell-gtk-embed.c

@@ -3,6 +3,7 @@
 #include "config.h"
 
 #include "shell-embedded-window-private.h"
+#include "gtk-compat.h"
 
 #include <clutter/glx/clutter-glx.h>
 #include <clutter/x11/clutter-x11.h>