Remove manipulations from elementModel
- Refactor drop list component
- Use of initial values for audio and video playback time

Defining the validators not in the elements anymore, but when creating
the forms

This makes for cleaner imports.

Since the array of 'ClozeElements' can be empty, it must be checked

This removes all the classes and goes back to simple interfaces for
unit, pages etc and all elements. This makes creating them easier and
cleaner.
Things like specific needs when setting element properties and most
importantly compatibility handling is done by a special service module,
that knows all the elements and how to repair old their potentially
outdated definitions.