to make them available for new trigger element

- It will be used for trigger element

#586

Making element components unclickable is now applied to all child 
elements. This is necessary because some Material-components have child 
elements with specific pointer-events settings, which makes it possbile 
to interact
with them even though the parent is not clickable.

It is also now set via class on a parent element instead of the 
component itself.

#524

The color buttons now use outline instead of background-color for 
showing the selected color.

Also some alignment improvement and a hover marker for the select 
button.

- 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

#538

#536

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

#456

- Add Property "hasAutoHeight"
- Add Directive AutoHeight

#523
- Improve sanitizing of position margins

#511

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

#511

- 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

The logic determined the circumstances when min and max values were 
used.
Now they are passed as they are and the CSS ruleset does the rest.

In Player-context the constructors are to fail, when the given 
properties are not exactly as expected. Therefoore they are validated. 
When this validation fails, an environment variable is checked to 
determine if the element should be created anyway with defaults. This 
switch is off in player-context. But the Editor is not so strict and can 
create elements with incomplete parameters.

There is also a new VersionManager which checks given unit definitions 
for version-validity. Upon failing this check Player and Editor show 
error messages.
The Editor accepts a certain unit definition version, but shows a 
warning.

The Editor checks units for invalid references after loading. If any are 
found, they are removed and an info-panel is shown.