#656

#623

#448

- Prevent input of 'Dead' keys

- Toggling of keyboard and keypad are now async
- Keyboard and keypad animations are disabled for a short period of time

- Input field manipulation moves from service methods to new
  TextInputGroupDirective

Windows convertibles were not recognized as mobile devices by this check

#461

#461
- This property will be used for all input assistance presets
  in the future

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

- 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

- Refactor directive

- Basic implementation

Also restructure files containing interfaces.

- Set the duration of highlighting anchors to 15 seconds
- Disable simultaneous highlighting of anchors

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

- Replace Material Droplist with native HTML events
- Remove simple-drop-list element; Cloze elements now use the normal 
DropList element
- Add example units

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