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

- 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

This is for the different use saces of a simple text element and a cloze 
element. The latter needs more inputs for child elements and also return 
JSON instead of HTML.

The NodeViews are for the TextEditor to render Angular components, which
now serve as preview instead of the old backslash escaped characters.

All extensions are now placed in a specialized directory. Except the
NodeView stuff which is kept together.

Was missing the actual change of the variable.

Also improve tooltips.

- Fix reading values from combinedProperties object. This object has the
needed property in its root.
- Don't show alignment buttons in dynamic mode
- Make the small input fields a little wider so the placeholder text
does not overflow, when there is no valid value to show. This may happen
when multiple elements are selected, which have differing values.

- Move the method to the unit service, so the proper 
updateElementProperty method can be used.
- Also improve the logic, reducing variables.

Only select the element component when it is not already selected. This 
prevents delesecting elements which have been multiselected.

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.

Correct german form: 99-66

This creates a blockquote-element, which is styled globally with 
specific quotes and alignment.

Images are always inline and scaled down to the font size of the text 
element as a whole.

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.

Use a suffix button to activate color picker instead of having an extra 
form field.

Also add direct input for likert line coloring color.

The theme file is now project specific because Angular demands assets
to be part of the project.
It also differs in content: The editor theme is only applied to canvas
elements. For the player every checkbox and radio button is affected.

The components now again use the default color of 'accent'. The custom
theme now has the green as accent color and no longer primary.

This new setting makes child items space around in a row with line 
break.

Images in vertical alignment now always reserve the whole row and don't 
appear next to each other.

Use unit field of UnitService directly to detect changes.

Variable naming, reorder imports etc.

Introduce interfaces and clean the structure.

This makes for cleaner imports.

The dropListList generation is now done when things are changed via the 
section menu and no longer on element update via unit service.
Also remove some interfaces to avoid circular imports. The interfaces 
are so small and insignificant that their form might as well be 
specified at usage point.

No longer have unit as subject. Components can use the unit variable as 
is and still get updated.
This removes some listener stuff in conected components.

Was not correctly creating new instances of non-primitive values. Now
uses "JSON.parse(JSON.stringify(element))" for deep copying the element
first.

Also refactor by moving the method to the UnitService, where all ID
handling should take place.

No longer is a singleton but a normal servicce now again. The singleton 
logic was needed in the past but not anymore.

Generally only the editor needs to generate new IDs. Before there was
need to have it in common because some elements (like compound elements)
needed to generte new Elements including IDs.
Now all ID stuff is handled by the UnitService and the elements classes
are handed the fully built elements. Unfortunately this means that some
logic has to move back from classes into the UnitService. To un-bloat
the UnitService a (and in the future some more) helper class is created
for this logic.

The most important thing is, that the UIElement no longer generates
an ID when created. The ID has to be created before and passed to the
constructor.

Now correctly sets the chosen background color and additionally sets 
another (configurable) color to the selected value.