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

- Improve variable naming for unit definiton type. (version string)
- Use 'iqb-aspect-definition@1.0.0' as value which better communicates
  the purpose of the data.
- Also update the HTML files with the new definition. (Even though it is
  not checked anywhere.)

Before only the 'parts' parameter was saved. Now the HTML text is saved 
as well, so future versions can properly restore the element.

... a played audio file

For compound elements, the form has more than one control in its
element array. These must all be checked for their valid state.

- Explicitly set the value of the form control of CompoundChildren,
because they are not yet fully registered in the onInit phase.
- Use ChangeDetectorRef to prevent
ExpressionChangedAfterItHasBeenCheckedError

The copy is needed to not have cross references between elements.

See 5ff73e14

This reverts commit 93e62a4a.
Teststudio does not accept verona 4 specification data

- Do not check whether pages or sections are valid
- Check the entire form and if necessary the controls

- Check if the selection refers to the content of the text component

- Remove the manipulation of the end offset
- Add the end container of the selection to the list of the selected
nodes if end offset is 0

This dash is a bit shorter.