- and AnswerSchemeValue with VariableValue

- Remove unused imports

#599

- Now every element can overwrite 'getAnswerScheme' of 'UIElement'

- Method is already defined in superclass

#583

#596

- 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.

#558

The ClozeDocument was not cloned properly. Since it is possibly 
infinitely nested, manual reconstruction is less viable.

- Remove all Roboto files

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

#506

The individual rows are considered as a whole and are therefore
adjustable as a whole via the parent element

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

#456

- Add Property "hasAutoHeight"
- Add Directive AutoHeight

#506

#452