[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)

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