* Remove FormService Injection
* Move FormService and form types to player
* Move interface ValueChangeElement to uI-element.ts
* Use interface InputElementValue instead of
string | number | boolean | null

No longer is async. For elements which need upfront source info (media), 
the media prompf is done in the unit service. Here we can keep it 
simple.
Also remove coordinates from factory as this is information that does 
not need to be known here. This is also handled in the unit service 
where applicable.

There is now a default method working the same as before. But elements 
(likert for now) can overwrite it and add some logic for specific 
properties.

This makes it easier to define types of objects in surrounding code.

This keeps import cycles from happening.

This makes checking the existence of properties for the property panel
possible, since it is possible to check for null but not for undefined.

- Remove interfaces
- Add classes for all elements as well as unit, page and section
- 2 intermediate interfaces are kept. They dont constituate a viable 
object but only additional properties, like background-color. Those can 
be added as interface to all elements. Initialisation has to be done by 
the element itself though and not by some constructor higher up.