New function which implements "smart" separators by iterating once over

New function which implements "smart" separators by iterating once over

	* gtk/gtkuimanager.c (update_smart_separators): New function which
	implements "smart" separators by iterating once over the entries of a
	menu, hiding and showing separators as necessary.
	(update_node): Mark separators used as fences of placeholders as
	hidden. Explicitly added separators are marked as smart. Call
	update_smart_separators after updating a menu or toolbar node.
	Connect update_smart_separators to "notify::visible" signal on menu
	and tool items.

	* tests/merge-[12].ui: Test smart separators.

	* gtk/tmpl/gtkuimanager.sgml: Add a paragraph about smart separators.