Better separation of concern of SCXML Workflow execution context and data

SCXML Workflow executor currently uses a SCXMLDataModel object to provide both context and data to the SCXML statemachine.
Functionally though, these can better be separated, and thereby also easier customized and implemented by specific workflow implementations (like DocumentWorkflow).
This also will make it easier to explain and document how the SCXML workflow works and can be implemented/customized.