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

#547

- Remove all Roboto files

Since there is just one font, this setting can be safely ignored. This 
is easier than sanitizing all elements.

#529, #530

- Changes were required due to material update

#528

#461

#461

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

#397

#520

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

#525

#511

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.

- Remove SanitazationService from player
- Extend interface ButtonEvent

#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