gtkmenubutton: Add menu button widget

As used in Totem and gnome-contacts. The widget
takes either a GtkMenu or a GMenuModel to construct
its menu, and can be given a parent widget to use to
position the drop-down (as used in GtkMenuToolButton).

https://bugzilla.gnome.org/show_bug.cgi?id=668013

gtk/gtk.h

@@ -128,6 +128,7 @@
 #include <gtk/gtkmain.h>
 #include <gtk/gtkmenu.h>
 #include <gtk/gtkmenubar.h>
+#include <gtk/gtkmenubutton.h>
 #include <gtk/gtkmenuitem.h>
 #include <gtk/gtkmenushell.h>
 #include <gtk/gtkmenutoolbutton.h>