#608

- Prevent input of 'Dead' keys

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

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

Their values are mapped by the ElementModelElementCodeMappingService

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.