SymbolicXMLBuilder

This class builds instance of Tree that represent XML.

Note from martin: This needs to have its position info reworked. I don't understand exactly what's done here. To make validation pass, I set many positions to be transparent. Not sure this is a good idea for navigating XML trees in the IDE but it's the best I can do right now. If someone who understands this part better wants to give it a shot, please do!