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-embedded-window.c

@@ -6,6 +6,7 @@
 #include <clutter/x11/clutter-x11.h>
 
 #include "shell-embedded-window-private.h"
+#include "gtk-compat.h"
 
 /* This type is a subclass of GtkWindow that ties the window to a
  * ShellGtkEmbed; the window is reparented into the stage