More work on the class browser; I had fixed it up to allow updating from the duchain, but David suggests I re-check the code model in that instance, which sounds like a good idea.

Committing this now (with that path disabled) in case I ever want to retrieve the code.

Also:
- track which files are referenced (by projects, the editor) and show while still referenced.
- cleanups
- remove old duchain-based sorting code
- make the encountered tracking code work on the top node for one specific file at a time
- some documentation (soon to be out of date ;)
- move node class to cpp file (private detail)