- Use own StateVariableStateService which uses the same new base class
as UnitStateService

- Basic implementation

Also restructure files containing interfaces.

This was necessary because the new text selection action of the button
is not a navigation.

- Areas are marked with the surrounding 'aspect-anchor' tag

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.

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

Their values are mapped by the ElementModelElementCodeMappingService

for type text and image

- add unit-definition-menu module
- add verona module and move verona services and verona interfaces to it
  to make them available in different modules
- move the distinction of different page types to app component to make
  them available for unitDefinitionMenu component

Change import paths of CommonModule files in editor and player

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