This has been refactored already. Those were not cleaned up properly.

Before the defaults were simply set on any field regardless of the 
passed object.

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.

There are still some problems. It works a lot better than before. Adding 
this as an intermediate solution.

- 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.