- editor and player have their own services to handle the errors

- to prepare markable support for text as compound children

#606

#605

Replace the assignment of the ElementComponent with the assignment of
a ElementComponent that will work in future versions of Angular

- Prepare usage in editor
- Remove test from ScrollPagePipe, because the call to "new Page" expected
  the initialization of GeometryComponent

- Add cdk scrolling strategy to keyboard that brings overlays to
scrolling

- Use the VeroSubscriptionService as a pure communication service
- Do not contaminate the VeroSubscriptionService with holding data

This button helps the user to detect if a page has additional,
hidden content

SectionVisibilityHandlingDirective now controls the visibility of
sections that depend on an audio or video

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

Delete keyboard module

- 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