- 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

Geometry: Add property trackedVariables

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

- InViewDetectionDirective determines the container itself
  as the previous binding returned an undefined value

- Resolve the conflict between the browser behaviour in concat scroll
  snap mode and the scroll button with delays
- Add space at the bottom of each page to ensure that the user
  can reach every element

- add padding
- add stopping on every page
- change selected index recognition

#579

SInce this is a non-Material element the font is not automatically 
inherited. Needs to be done explicitly.

#574

- Minuend instead Subtrahend is strikable

#571

#578

An error was made in the last release. Now the numbers are correct.

#574

#560

style used in other cloze children causes the error
':host {display:flex !important; width: 100%; height: 100%;}'

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

#558

#559

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

#562

#542

This is much smaller and the intended way for web apps.

#457
#539

The directive for dynamic backgroudn color was outdated.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

#561