Request to add *pre-transition* functionality to workflows.

As requested by customer:

The needed functionality to allow workflows which can enforce important guidelines requires two mechanisms:

1. Add a new Validator type "Field check" for Workflow Transitions
   • The Field check validator lets you choose one field which should be checked
   • The transition is only performed if the field is not empty
2. Allow "Pre Functions" just like Post Functions.
   • It should be possible to assign a value to a field (update issue field)
   • Lots of flexibility would come if it would be possible to copy the value from another field (also useful in Post Functions), allowing to combine hidden fields with this functionality