- and AnswerSchemeValue with VariableValue

- Remove unused imports

#599

So it moves away from the "normal" settings.

- New property group for flexible layout with more options to change the 
behaviour. It is now possible to build all other operations in this 
preset.

This is a free operation type that does not render much.

#590

#588

Small cells and canBeCrossedOut are property of the row as they are all 
cells or none.
Additional property 'is2DigitHelperRow' is for different types of helper 
rows: single-digit in no crossing out or 2 digits with crossing out. 
2-digit is only for the top 2 substraction rows.

#589

This holds variable names, taken from the GeoGebra applet, and uses 
those variable values in the value. This makes result analysis easier.

-> 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