Make the QuickFixOperation interface more useful.

* Rename cursor(unsigned/AST*) to selectToken and selectNode.
  These return a cursor that has the given token or AST node selected.
* Rename getTokenStart/EndPosition to tokenStart/EndPosition.
  Instead of returning line and column numbers, return a position
  usable with text cursors.
* Remove moveAtStart/EndOfToken.

Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>