This method may only be called if gmenu_tree_iter_next() returned GMENU_TREE_ITEM_DIRECTORY.
This method may only be called if gmenu_tree_iter_next() returned GMENU_TREE_ITEM_HEADER.
This method may only be called if gmenu_tree_iter_next() returned #GMENU_TREE_ITEM_SEPARATOR.
Change the iterator to the next item, and return its type. If there are no more items, %GMENU_TREE_ITEM_INVALID is returned.
This method may only be called if gmenu_tree_iter_next() returned GMENU_TREE_ITEM_ALIAS.