- add padding
- add stopping on every page
- change selected index recognition

- Replace throwing Errors with InstantiationErors in unit, page and
  section in strictInstantiation mode

#547

#397

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

#401

- Only elements with this property are considered in the player when
determining the presentation state
- Add the registration of CompoundElements at the UnitStateservice
- Fix visibility handling of section with simple rules

- Add translations for operators
- Improve and extend the evaluation of visibility rules

- Refactor directive

- Basic implementation

Displaying a message is task of the host
#398

- Using scrollIntoView with target 'center' was not always reliable
  especially in FF

- The workarounds for ff with setTimeout are no longer necessary in the
  new version and have been removed
- Replace ChangeDetectorRef.detectChanges with setTimeout because the
  scroll button was not always reliably displayed at startup

- A HotspotImage has an image and divs placed on it, which are used
as hotspots
- Inherits from FormElementComponent, which is why the hotspots are kept
  as a list in the value property.
- Can be validated as required and uses its own custom validator

Fix import order

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

- Refactor subscriptions for verona commands

- Fix related tests

- Rename methods with prefix 'on'