This removes all the classes and goes back to simple interfaces for
unit, pages etc and all elements. This makes creating them easier and
cleaner.
Things like specific needs when setting element properties and most
importantly compatibility handling is done by a special service module,
that knows all the elements and how to repair old their potentially
outdated definitions.

Now correctly creates child elements for every marker it finds.

Updated the TipTap extensions to use model instead of just the ID.

Also improve interface naming.

- 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.