Create a wid specific MVELEvaluator

Currently workitem definitions are evaluated by RawMVELEvaluator which sends the wid mvel expression as-is to mvel for evaluation.
Creating wid specific evaluator would allow to not run into issues with package changes (imports have to change) and not force users to define imports in wid at all. Future extensions of the wid format (making it simpler) would also be possible.