Frames are supposed to be used to group other elements and therefore 
usually needs to be behind them.

- Remove element as parameter

Compound elements are now supposed to use overlays for their child
elements. This overlay makes selecting child components (by clicking on
them) and marking them as selected (done by the selection service)
possible.
The SelectionService no longer needs special logic to handle compound
children selection, as they now also have an overlay with the same
interface as normal (canvas) elements.

A few modifications in connected directives are necessary. The now
handle children components in a proper array instead of QueryList.

Likert elements do not have clickable children yet and work a little
differently. This should probably be unitized in the future.

- Implement marking buttons as toggle buttons

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

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).

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.

- 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

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

This disables the possibility of nested markers

This makes the list not collapse when used in dynamic context.

Those are wanted by the product owner.

This complicates styling and is somewhat useless as the border can also 
be removed by setting the border-width to 0.

Elements are supposed to be centred when this setting is used. Since it 
is the same CSS for almost all element components it is put into global 
styles.

Missing components: video, spelling.

This reverts commit 87534c53.

Outline may be shown when valid dropLists are highlighted. The bottom 
patr of the outline is cut off when the whole width is used by the 
dropList.

Section was picking up the size of children elements. With relative it 
works.

This allows elements in dynamic sections to have fixed dimensions.

Some modifications to property panel logic, to show dimension fields and
proper labels.

Only implmented in dropList and button elements.

The child elements are created directly in the component when reading a 
serialized cloze.

The whole parsing process of cloze elements will be reworked soon. So 
this probably is a temporary fix.

Options still had a margin-top, which is wrong when there is no label 
above.