Prepare QtHelp library for modularized documentation.

- Support ".." in QCH documentation file paths.
  This essentially means that we stop using cleanPath, and where needed,
  prepend the virtual folder name, followed by "/..", in front of the
  original path.
- Remove com.trolltech default DocPath since this was not used anymore.

Done-with: Casper van Donderen <casper.vandonderen@nokia.com>
Change-Id: I7309805000f343e2f58c74cd0655a936399fd939
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Reviewed-by: Casper van Donderen <casper.vandonderen@nokia.com>