Source reference should be set in XDOM before transformations execute

Transformations may need to use the source reference.

Actually I think we could go one step further in the future and do this:

• Introduce a Rendering Context
• Set the source reference in the Rendering Context
• Pass the Rendering Context to Parser.parse()
• Modify the Parsers so that they take the source reference from the Rendering Context and set it in the returned XDOM