Based upon the is_in_state selector, propose a more robust selector about workflow transition.
Excerpt from the commit message:
- add new on_transition selector
Especially useful to match pending transitions to enable notifications when
your workflow allows several transition to the same states.
Note that if workflow change_state adapter method is used, this selector
will not be triggered.
- In debug mode:
These both selectors will check against the entity current workflow if expected
values given in selector argument are valid.
ValueError exception will be raised for unmatching state/transition
names against the current workflow (generic etype workflow).
(check against custom workflow is not implemented)