- Map value between unit state and element model
- Takes over methods that were previously present in element container
- Gets and holds a list for mapping drop list elements

Fix a problem when saving compound children

The entire HTML is no longer saved, but only the indices and the color
of the markers

Add the missing property playerProps

If the virtual keyboard with numbers (or with numbers and operators) is
used, input via the normal keyboard is also restricted to these
characters.

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.

The presentation will not be completed until all audios with the
minRun property have been played.

Since CompoundElementComponent now inherits from ElementComponent,
a case distinction can no longer be made implicitly via the property
domElement.

Split code into separate methods

Split the code into separate methods

The MarkingService now takes care of the selection and its validation.
The text component sends its change via elementValueChanged.

All elements now use the elementValueChanged event. Also
formValueChanged is changed.

Since the options of the DropListComponent are now managed as values,
the DropListComponent can be treated as a standard FormElementComponent

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

Use the status of each element

* Add magnifier properties to `ImageElement`
* Set used value at `UnitStateService`
* Bypass parsing error of distpacker

* Refactor markable text
* Prevent multiple selections (feature is only supported in Firefox)

* Add IntersectionService
* Add CompoundElementComponent as parent for LikertComponent
* Inject ElementRef to common ElementComponent
* Rename players' ElementComponent to ElementContainerComponent

* 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

* Add `InputElementValue` as type when needed

Should at least be in util folder. And since it is imilar functionality 
to the unit factory it is placed within.

Make sure that selection start is smaller than selection end