- Implement marking buttons as toggle buttons

- 'hideOtherPages' is not supported by verona api
- 'uninterruptible' is already fulfilled by disabling pause and
progress bar

Name changed during development

Do not consider all form elements, but use a list containing only form
elements with validators

Refactor MathKeyboardComponent to reduce code size

Add styles for fixed right position
Customize markup for different positioning
Refactor Usage of KeyboardService in components

Was not working at all before. Now the logic is much simpler with the 
use of tiptap attributes.

Is now able to interpret more stuff from the editor. The only thing 
missing are the list styles.

The default value of 1 MB was too low for the player, which currently
requires 1.18 MB.

Fixed-width pages are always centered in the browser, even if the
specified percentage is not equal to 50%.

The percentage weighting of the pages has an effect if the width of the
pages is greater than the width of the browser window.

Was broken after previous refactoring.
The renderHTML method only works with a proper return. Don't know why. 
Therefore disregard the linter warning for now.

Cloze no longer has a text property and needs a way to open the 
TextEditor.

Now manages it's width and height itself.

Also has some new properties:
- verticalOrientation should be self-explanatory
- dynamicWidth allows the user to set a width manually. The buttons on 
the element automatically stretch to meet the set width (or height).

- Naming
- Focus after nodeview insert

Also editor in cloze mode now respects the element's font size.

Cloze elements are displayed inline and have no way to show a label.

Now manages height and width by itself. So no overlay div is needed in 
the cloze component.

- Now only has a variable 'document'. This holds the HTML representation
  in an object. This object is enriched with element models.
- Because the the TextEditor extension can neither create multiple
  element instances nor use the IDService to generate their IDs, this
  has to be done afterwards. See ClozeParser.
- The cloze element has rather extensive compatibility handling because
  cloze elements used to save an actual HTML representation. This has to
  be transformed to JSON/object. Therefore we replace the old backslash-
  markers with custom HTML tags. The editor object does this
  transformation. It needs some custom extensions to recognize (and
  don't remove) the HTML tags though.
- Cloze now shows a placeholder text when empty
- The cloze component needs a small pipe to extract text formatting
  options from the paragraph parts.
- For getting the child elements for the player the models have to be
  extracted from the somewhat complex (JSON)document.
- Added some rudimentary interfaces for the TextEditor document format.
- Removed the old ClozePart interface. This is quasi part of the new
  interfaces.

- Give proper name
- Fix variable names
- Fix deprecated methods