- Dnd values now have a special type that has an ID, a string and
  another string for an image path.
- A new dialog exists for editing thoe options. Similarly to the likert
  header.

The interaction can now be turned off without hiding the button.

The problem being solved is that compound-sub-elements must not have
all the properties other elements have. For exampe positioning info.
Sub-Elements are positioned inline. To avoid having similar class
inheritance structures for positioned and inline elements, we use
interface. Positioning is done via interface and is therefore optional.

Those optional properties are kept in an object in a variable. This
allows to hide specifics from element classes and removes the need to
initialize all the fields in every class using the interface.
The elements mostly care about their own specific properties anyway.

The same technique is used for font and surface elements. Here we also
don't want to initialize all the styles in every class.

Changing values of element properties works like before. Just set the
property as if it would be a direkt prop of the element. The element
itself puts the value in the currect sub-object (positioningProps,
fontProps etc).

For reading the values there is a similar method, but it is not used
anywhere by now. Since the properties panel operates on the raw values
anyway (refer CombinedProperties) and for element components this would
mean calling a function in the template, which caused change detection
to run the function very often and is therefore bad practice.

Additional notes and refactorings:
- Restructure all files in common. UIElements in the same folder etc.
- Move all interfaces to the UIElement file. This avoids circular
  imports, which would be needed to allow PositionedElement to extend
  UIElement.
- Interface initializers also handle reading values in the old form.
- Add PositionedElement for use in canvas overlays. This is a UIElement
  with guaranteed postionProps.
- Don't export all Material packages from common, only the ones
  used in player and editor. The rest is only used in common and
  does not need to be exported.

Min-height should be optional for dynamically positioned elements, as 
you usually don't need it and it causes more problems than it solves. 
Elements with min-height have cause unexpected margins between elements.

Similar to a text element but parses the text on change an replaces 
markers with actual elements.
The text is then displayed inline with the elements.

The explicit check for undefined ensures that the value false of a
boolean property is taken from serialized elements.

Although the DropListElement is a FormElement, its value is represented
by the field options. Therefore, this state must be saved.

This element has a list of string as editable options. Those strings 
form draggable elements which may be reordered or moved between 
different DropList elements.

Also refactor option change logic to allow any property not just the 
text property.

This works similarly to the standard radio group, but has a fixed 
horizontal layout with images above the options.

For elements which don't have this functionality the workaround is to 
use pointer events and make the elements unclickable.

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

* 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.