* 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.