-> Don't hive line-coloring input but just disable.

#558

Explicitly define a label to be shown, since the shown label is done via 
innerHTML.

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

#548

#538

#536

- TextAreaElement, TextFieldElement, TextFieldSimpleElement,
  SpellCorrectElement:
  - remove property: softwareKeyboardShowFrench
  - new property: addInputAssistanceToKeyboard

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

#456

- Add Property "hasAutoHeight"
- Add Directive AutoHeight

#382

#525

#523
- Improve sanitizing of position margins

- Don't report empty reference string as invalid
- Add VideoElements to check

Not just when an older version is detected.

#511

GGB elements without toolbar aren't loaded, if this parameter is true

#511

- crossOutChecked adds line-through styling if checked. This is used in 
Cloze texts to cross out words.
- This means Checkbox is no longer a PositionedElement and does not 
validate PositionProperties

The ClozeDocument needs properly instantiated UIElements for the custom 
elements.
Instead of creating a new ClozeDocument on the basis of what TipTap is 
producing and replacing the custom parts with instances, the 
ClozeDocument is kept in place but the custom parts are collected into a 
list and changed there.

#450

The logic determined the circumstances when min and max values were 
used.
Now they are passed as they are and the CSS ruleset does the rest.

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.

#512

- Model constructors (elements, unit, page, section) now have a 
dedicated parameter, which makes sure all properties are set and valid. 
This way the player can instantiate models without having to interpret 
the values. Any interpretation is done only by the editor, which now is 
solely responsible for creating defaults/blueprints for new elements. 
Models can no longer be created by incomplete paramaters.

- Also refactor Stylings to have sub groups for common styles