- 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

#511

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

- 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

Instantiate the child elements coming from the editor (i.e. when the 
document changes). Otherwise the cloze children are all on the same 
object reference.

#513

- Can also be used with other elements

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