radio-menu-item: Add join_group()

The other Radio* widgets have this convenience method that removes the
memory management of the opaque GSList used to handle the group from the
API usable from language bindings (especially the ones not based on
introspection).

This commit adds gtk_radio_menu_item_join_group().

https://bugzilla.gnome.org/show_bug.cgi?id=671362

gtk/gtkradiomenuitem.h

@@ -95,6 +95,10 @@ GDK_AVAILABLE_IN_ALL
 void       gtk_radio_menu_item_set_group                     (GtkRadioMenuItem *radio_menu_item,
 							      GSList           *group);
 
+GDK_AVAILABLE_IN_3_18
+void       gtk_radio_menu_item_join_group                    (GtkRadioMenuItem *radio_menu_item,
+                                                              GtkRadioMenuItem *group_source);
+
 G_END_DECLS
 
 #endif /* __GTK_RADIO_MENU_ITEM_H__ */