add "submenu" property. Fix gtk_menu_item_set_submenu() to accept NULL

add "submenu" property. Fix gtk_menu_item_set_submenu() to accept NULL

2007-06-13  Michael Natterer  <mitch@imendio.com>

	* gtk/gtkmenuitem.[ch]: add "submenu" property. Fix
	gtk_menu_item_set_submenu() to accept NULL submenus and deprecate
	gtk_menu_item_remove_submenu() because we have a properly working
	setter now (bug #447065).

	* gtk/gtk.symbols: deprecate gtk_menu_item_remove_submenu().

	* gtk/gtkcombobox.c: use gtk_menu_item_set_submenu() instead of
	gtk_menu_item_remove_submenu().

svn path=/trunk/; revision=18125