- 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

- Only elements with this property are considered in the player when
determining the presentation state
- Add the registration of CompoundElements at the UnitStateservice
- Fix visibility handling of section with simple rules

- 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.

- Swap the position of dialog buttons

- Swap the position of dialog buttons

- Add translations for operators
- Improve and extend the evaluation of visibility rules

This hack allows the page tabs to have a pseudo tab to create a new 
page.
This tab is disabled so the user can not change to it, but is still 
clickable.

They are treated as containers by the player and therefore have no
values that can be queried.

- Fix typo in css classname

- Basic implementation

Also restructure files containing interfaces.

- Add ReferenceManager to check if deleted elements are referenced 
anywhere in the unit. If so display a warning.
- Move page logic to UnitService

- Background color must not be transparent for the stickied panel to 
overlay properly.

- Use TextImagePanel where possible

Toolbar is a single word, therefore any capitalized letters are errors. 
No need to sanitize, since this was broken anyway.

#384