answer domain improvements:
* add answer domain age
* add answer domains to all question elements
answer domain improvements:
* relations needs to be on 'parent' class, not on inheritance class
Adjust edit question form to hierarchical model and new insights:
* form/question/properties.php: main form, uses
* form/question/Question.php and
* form/question/AdminAnswerDomain*-classes to create the correct form based on answerDomain Type
* model/AnswerDomain*::getAvailablePresentationMethods return array with available presentation methods per answerDomainType
improved i18n of some strings