This facilitates the testability

- Reduce amount of properties
- Split code in separate methods
- Define some properties as input properties

This eliminates the need to specify the different page types in
AppComponent

Knowledge of the 'document' existence is only a matter of the function 
not the caller.

Use "display:none" instead of "*ngIf" to hide the section and make sure
its children are loaded.

- Remove TODOS

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

Use the mapper method `toUnitState` of the UnitStateElementMapperService
when registering elements and their initial values

Use configuration and refactor keyboard template

This is possible since now playerState also has the information of the
underlying scrollPages

- 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

Overwriting styles of HTML elements can lead to unwanted side effects

The height of an empty droplist can't be calculated on ios 14
because it's in a flex container

Change import paths of CommonModule files in editor and player