[selectors] modify workflow selectors: is_in_state, on_transition
- factorize `is_on_state` selector
- 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)
