* demos/gtk-demo/toolpalette.c (load_special_items): Demonstrate
	gtk_tool_item_group_set_label_widget().
	* gtk/gtktoolitemgroup.[h|c]: Rename the "item-group-name" property to
	"label" because that is what it is. Likewise rename the functions.
	Add a "label-widget" propert and get/set_label_widget() functions,
	based on the same code/API in GtkExpander.