https://bugzilla.gnome.org/show_bug.cgi?id=741610
* @model: the model to flatten
* @with_separators: if the toplevel should have separators (ie: TRUE
* for menus, FALSE for menubars)
+ * @merge_sections: if sections should have their items merged in the
+ * usual way or reported only as separators (which can be queried to
+ * manually handle the items)
* @action_namespace: the passed-in action namespace
* @insert_func: insert callback
* @remove_func: remove callback