#656

The delay is used to check whether a selection is still available after
PointerUp, as this is often deleted in Firefox under Windows mobile

#623

#613

#552

#608

- 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

- Remove unused imports

#596

#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

#525

In Player-context the constructors are to fail, when the given 
properties are not exactly as expected. Therefoore they are validated. 
When this validation fails, an environment variable is checked to 
determine if the element should be created anyway with defaults. This 
switch is off in player-context. But the Editor is not so strict and can 
create elements with incomplete parameters.

There is also a new VersionManager which checks given unit definitions 
for version-validity. Upon failing this check Player and Editor show 
error messages.
The Editor accepts a certain unit definition version, but shows a 
warning.

The Editor checks units for invalid references after loading. If any are 
found, they are removed and an info-panel is shown.

- 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

- Basic implementation

Also restructure files containing interfaces.

Displaying a message is task of the host
#398

Uses external software 'MathLive'.

Distpacker issues with mathlive have been solved in a custom 
distpacker.ts in scripts-folder. This is now used instead of the 
distpacker from the npm package.

This was necessary because the new text selection action of the button
is not a navigation.

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

- If this property is set, the input of more characters than specified
in maxLength is prevented
- It also prevents the insertion of text.