- rename variables, methods and properties
- reduce attributes for methods
- split code in sun methods

This way elements can handle their logic themselves without having to
rely on outside utility classes.

Also restructure files in common in a more logical way.

This facilitates the testability

elementModels are described in the unitDefinition
elementCodes are saved in the unitState

Their values are mapped by the ElementModelElementCodeMappingService

for type text and image

- Rename class name, methods and properties
- Remove method registerDropListValueIds
- Calculate array of DragNDropValueObjects in app component

Change import paths of CommonModule files in editor and player

Replace custom pointer events with native pointer events

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 helps to divide the code of the element container into smaller
meaningful units.
In addition, each element can be given its own form group.