Adapt model classes

This MR proposes minor changes to the model classes:

• All type classes were defined to be abstract. Removed abstract keyword where necessary.
• Fixed ValidationProperty to be inline-or-by-reference.
• Added missing ADEObject interface to type classes.
• Made sure that parent references are correctly set on child elements. This is required for traversing the type hierarchy bottom-up.
• Added type class for the ADE hook property validationResult.

One additional remark: There are property classes for all error classes. For example, there is a HoleOutsideErrorProperty for the error HoleOutsideError. However, when looking at the XSD, I think only the AbstractErrorProperty is really used (by ValidationResult). So I guess, all other error property classes could be safely removed.