#612

This makes the controls fit better into the fieldset box.

#382

- 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 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.

- Model constructors (elements, unit, page, section) now have a 
dedicated parameter, which makes sure all properties are set and valid. 
This way the player can instantiate models without having to interpret 
the values. Any interpretation is done only by the editor, which now is 
solely responsible for creating defaults/blueprints for new elements. 
Models can no longer be created by incomplete paramaters.

- Also refactor Stylings to have sub groups for common styles

- 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.

Several elements were broken after updating the Material library.

Add new component for combobuttons, which allow selecting and applying a 
value in one connected element.

New character: protected dash (which does not break lines)

#455

- Set the duration of highlighting anchors to 15 seconds
- Disable simultaneous highlighting of anchors

The extension is needed to define text areas in the rich text editor
and make them available for the button component

...but disable the list auto-generation for bullet and ordered lists.

#226
#369

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

- Also simplify the button constructor

- Make position props on buttons optional
- Move UIElement creation back to Factory instead of within Section

#294

Now the dialog scrolls and the buttons at the bottom can be reached even 
on low viewport height.

#294

The TipTap editor needs a setting to be able to read base64 encoded 
images.

Also improve method name.

#297

#298