Added `subtasks_by_task` query for tasks.

Conflicts:
	opengever/globalindex/tests/test_query.py
Added `correspond_to` check for all actors.

Used by task transitioncontroller to handle task-assignments the same way, whatever
it's and user or an inbox which is assigned.
Completly rework Task Transitioncontroller guards:

 - introduce Conditions helper class
 - Define complete and sensefull tests (dropping Mocktest's)
Actionmenuviewlet: display agency actions, in a separate dropdown button.
Rework inboxtransitioncontroller.

Conflicts:
	opengever/inbox/tests/test_transitioncontroller.py
Pep8 cleanup.
Dropped no longer used transitioncontroller helper methods.
Reworked forwarding test.