ToolItem.addOnCreateMenuProxy

This signal is emitted when the toolbar needs information from tool_item about whether the item should appear in the toolbar overflow menu. In response the tool item should either call gtk_tool_item_set_proxy_menu_item() with a NULL pointer and return TRUE to indicate that the item should not appear in the overflow menu call gtk_tool_item_set_proxy_menu_item() with a new menu item and return TRUE, or return FALSE to indicate that the signal was not handled by the item. This means that the item will not appear in the overflow menu unless a later handler installs a menu item. The toolbar may cache the result of this signal. When the tool item changes how it will respond to this signal it must call gtk_tool_item_rebuild_menu() to invalidate the cache and ensure that the toolbar rebuilds its overflow menu. TRUE if the signal was handled, FALSE if not