Before it was just the bottom helper row.

#592

- 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

#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

- Minuend instead Subtrahend is strikable

#571

#578

#560

The former solution was affecting the selection CSS of the button. Now 
the text is moved down instead.

#562

The directive for dynamic backgroudn color was outdated. Directive is 
removed and problem is solved via CSS custom variable.

#561

Was completely broken: not showing at all in Editor, not clickable in 
Player.

Removed pipe 'aspectInputBackgroundColor'. Replaced by more elegant 
solution via inherited background-color.

THis component was still somewhat broken after the Material update.

#541

#545

- Remove all Roboto files

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

#533
- Remove unnecessary and wrong setting of underline property

#532
- Replace the global label identifier

#535

#456

- Add Property "hasAutoHeight"
- Add Directive AutoHeight

#520

- 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

#521

#521

#521

#521

- Replace or remove no longer used material customization

#521

- Remove no longer used material customization

Also restructure files containing interfaces.