related to #1908756: Separate Action Links (MENU_LOCAL_ACTION) from hook_menu()
For tabs we need to have hierarchy which distinguishes them from local actions.
A knotty problem to be solved is also the _to_arg functionality which is used in tracker module and other places. The pattern for solving this would probably apply to both tabs and local actions. Basically we need to be able to call a function to populate a value when constructing a synthetic path to make a request object.