menuitem: port to gadgets

This is not a complete conversion yet - more changes will come in later
commits.

gtk/gtkmenuitemprivate.h

@@ -21,6 +21,7 @@
 #include <gtk/gtkmenuitem.h>
 #include <gtk/deprecated/gtkaction.h>
 #include <gtk/gtkactionhelper.h>
+#include <gtk/gtkcssgadgetprivate.h>
 #include <gtk/gtkcssnodeprivate.h>
 
 G_BEGIN_DECLS
@@ -40,6 +41,7 @@ struct _GtkMenuItemPrivate
   GtkAction *action;
   GtkActionHelper *action_helper;
 
+  GtkCssGadget *gadget;
   GtkCssNode *arrow_node;
 
   guint submenu_placement      : 1;