Made buildable and added support for adding children of type "submenu"

Made buildable and added support for adding children of type "submenu"

	* gtk/gtkmenuitem.c: Made buildable and added support for adding children
	of type "submenu"

	* gtk/gtkwindow.c: Added support for custom tag "accel-groups" to add GtkAccelGroups
	to the window.

	* gtk/gtkcontainer.c: Added builder contextual warnings in buildable_add_child()

	* gtk/tests/builder.c: Added tests for buildable menus (test that accelerators are
	properly connected on stock items, test the menu hierarchy, test permission to
	add alien/custom menuitem children).

	* docs/reference/gtk/tmpl/gtkbuilder.sgml, docs/reference/gtk/tmpl/gtkwindow.sgml,
	docs/reference/gtk/tmpl/gtkmenuitem.sgml: Updated docs for buildable submenus
	and accel groups.

svn path=/trunk/; revision=21767