Move KeyInputModule to the modules subfolder
Create template file for AppComponent

Rename properties and events for keypads

Rename KeyboardComponent to KeypadComponent
Rename KeyboardService to KeypadService
Split LayoutComponent into LayoutPlayerComponent and
LayoutPagesComponent
Prepare the introduction of an additional keyboard

see commit #866a33c6

Replace custom pointer events with native pointer events

By default, text fields and areas only allow the input of keys from the
respective keyboard.

and in ElementFormGroupDirective

This is necessary because the sanitizer returns now an array instead of
the modified unit definition.

Reduce the number of input parameters in FloatingKeyboardComponent
Move double-used method to KeyboardService

Instead, use Textfied component with the property isClozeChild

to avoid lint errors in templates

Rearrange the order of function calls

Replace the direct method calls in media player components with events.
For this purpose, expand the inputs of the media player components that
are defined in the ElementMediaPlayerGroupComponent.

belongs to commit #f6668bdd

- Units and element are now built via the factory. This ensures only the
  wanted properties are present. All others are discarded.
- Since the sanatizer is supposed to check the unit defintion version
  it is used by player and editor and therefore put in common.
  This in turn makes it necessary to put the JSON resolver settings
  in the root-TSConfig file.
- Renamed a couple of properties to have clearer names.

- Verification is task of the common sanitizer
- Remove unnecessary AlertDialog

- Reimplement UnitStateComponent as central point to send the collected
information
- Implement ValidationService to determine the responseStatus of all
forms

This was necessary after the surrounding element container with its
styles and classes was deleted

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.