#612

- crossOutChecked adds line-through styling if checked. This is used in 
Cloze texts to cross out words.
- This means Checkbox is no longer a PositionedElement and does not 
validate PositionProperties

- Set the duration of highlighting anchors to 15 seconds
- Disable simultaneous highlighting of anchors

The extension is needed to define text areas in the rich text editor
and make them available for the button component

...but disable the list auto-generation for bullet and ordered lists.

#226
#369

- Make position props on buttons optional
- Move UIElement creation back to Factory instead of within Section

The TipTap editor needs a setting to be able to read base64 encoded 
images.

Also improve method name.

#297

#298

There was at least one bug, where entering a number could create a list 
with wrong styles. This is the easy fix by disabling all that, instead 
of fixing it, which might bo out of our jurisdiction anyway.

#226

This way elements can handle their logic themselves without having to
rely on outside utility classes.

Also restructure files in common in a more logical way.

since the player uses it as well

Set back to the default setting to prevent CSS settings to spill over 
into other parts of the application.

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outdated definitions.

This is done according to the Angular best practices and minimizes
namespace collisions when using other packages which might use the same
selectors.

Technically the selectors are not even becessary since we use an Angular
component factory to create element components but it is still useful
for the DOM element naming, for debugging purposes for example.

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

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.

- Naming
- Focus after nodeview insert

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

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.

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 controls the general font size. Specific spans can still be styled 
individually in the text editor.
- The TextEditor also uses this font size as default when rendering the 
text.

The font size is applied by setting the list style. This is a 
workaround, since the list extension does not support setting font size 
directly. It takes the selected font size (which is now saved ina 
variable) and applies it. This is indepent of the font size of the text 
in the list. The item font size is done individually with a span anyway.

Also font size and list style droplist now (re-)apply the value on click 
of an item. This allows updating font sizes and list styles without 
having to select another option first.

- Add a new variable to save the selected font size and reuse it for 
list styles.

(when not set)

- Reordered the buttons to make it look nicer (but still make sence 
logically)
- Put some more divs with flex layouting to prevent elements moving 
unexpextedly.
- Changed color and list inputs to split-buttons. Those compone a button 
with a dropdown.
- The editor now saves the set color and  it can be applied to 
subsequent selections as well.

Before it was done with a dysfunctional hack. Now with proper extensions 
to existing (tiptap-node-)extensions.
Also reads existing style and sets select-element accordingly.

The main components are right at top level and the directories below
mimic the data structure (unit->page->section).
The toolbar is only used in dev mode and does not belong directly to the
unit view.
The TextEditor is a complex component and deserves it's own directory
even though it is used within the unit-view. Might be refactored to its
own module later.

Customized margins for paragraph elements.

Mistake while merging earlier.

The other one used some deprecated functionality. Also we have more 
control over what is happening now.