This is only used by the Editor. It was in common while the 
SanitizationService was used by the Player which then could display 
errors. Sanitization has been refactored and this is no longer needed.

This way it can be used for testing.

This helps accessibility in general and specifically allows testing for 
it.

#511

This whole class will be deleted soon.

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

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.

- Remove dynamicPositioning from PositionProperties. This is not 
knowledge that the element needs to know. In the few places it was used, 
it was fixed or solved by asking the containing section about which 
positioning is active.

- Fix and improve cloze child sizing. All can now have dynamic width and 
height.

- Fix empty lines in ClozeDocument rendering

- Editor SelectionService now knows if a cloze child is selected. This 
way the PropertiesPanel can show valid dimension inputs regardless of 
the underlying section.

Also restructure files containing interfaces.

causes problems after update

Measure holds the value and a unit for CSS properties.

- Add originListID and originListIndex to all DragNDropValueObjects

- allowReplacement (moved from element to list prop)
- originList and Index are non-optional (means placeholders have to be 
used sometimes)
- Re-introduced the dict of all DropListComponents. This is the only way 
I have found to get a reference to a DropListComponent by ID. This is 
needed for returning to origin. Origin is not actually part of the 
transaction and needs to be fetched from somewhere else. Hence the dict.

- Console statements still present for debugging.

- Add 'Button' as possible child of cloze elements

- Replace Material Droplist with native HTML events
- Remove simple-drop-list element; Cloze elements now use the normal 
DropList element
- Add example units

- remove ID handling from model classes. This is now done by the editor 
itself (mainly unit service).
- Rename and move IDManager to editor as IDService

Just called from the editor, before creating the Unit instance.
It goes through all IDs and checks for duplicates. If found it generates 
a new one and shows the prompt.

Transform string array to Textlabel array.

#338

had some issues with the switch to the new options stuff.

When calling the element constructor may generate IDs. Those appear as 
already registed later when the unit is instantiated for real.

- Sanitize without using unit definition version

- Add "isOlderThan" method to the "SanitizationService" to distinguish
old ToggleButtons with the property "option" from new ones
- Use SanitizationService.isUnitDefinitionOutdated in Player to set
the unitDefinitionVersion (should be modified!)

The equality check compares the reference of the array and not the
values of the array.

sanatizer
TextImageLabel:
 position -> imgPosition

DragNDropValueObject
 stringValue -> text
 imgSrcValue -> imgSrc

RadioButtonGroupComplexElement
 columns -> options

Likert
 columns -> options