Refactor collection structure

This update won't add any visible functionality. Everything should still behave as previously. But it implements abstract classes and a few implementations for the questions and the answeroptions.

So for example there is now a AbstractAnswerOption class and a DefaultAnswerOption class which extends the abstract class and implements the behavior of the current answeroptions.

An AbstractQuestion class is the super class for SingleChoiceQuestion, MultipleChoiceQuestion and SurveyQuestion classes.

This will allow to create very specific Questions and Answers with different behavior. Still there is lot work to do (none of these clsses are instanciated currently) but this update will change the application structure so it is better to split the implementation into several branches.