Some rewriting, TODOs.txt updated:

Some rewriting, TODOs.txt updated:
- Removed double $visitorInstance (BaseFrameworkSystem has already a generic
  instance/setter/getter)
- Some list classes does not implement getListIterator(), please use
  getIterator() instead, the default iterator is (as for now) fine here
- Moved code from constructor to factory method to minimize code in constructor
- TODOs.txt updated due to internal TODOs closed)