Add a possibility to map data constants for UI actions

At this moment when you have a UI action in an entity it is only possible to map a field from that entity or the entire entity. It would be useful if also a data constant could be used in the parameter mapping.

Or even expanding the functionality and instead of using Field there would be XPath which would enable also other functions to be used in the parameter mapping, similar to the context mapping for a workflow called from another workflow.