Aslo create new props panel component and move border stylings to 
separate fieldset (wihtin the styling tab).

#291

One as a general label above the table and one for the first column 
heading.

Since this is no proper InputElement separate properties panel inputs 
are used.

#272

Add new parameter 'itemsPerRow'. This limits the grid columns used, 
making items move to the next row if overextending.

Also simplify component structure.

Move label creation to ElementFactory.

No way has been found to have an abstract static method for 
OptionElement's label generation, therefore silence the linter.
Same for getElementComponent.

This is only useful in CompoundElements. So use checks for that class 
instance.

So TipTap does not moan about empty document.
Put placeholder text in the cloze document instead of the component 
template.

Now checks when creating or updating object if IDs are okay.

Initialize to null, to be able to differentiate in the props panel.

Also improve props panel layout by always showing all fields and 
disabling them if needed instead of hiding them.

Fix translation.

#318

#293

#283

Also refactor properties panel and ToggleButton class logic for striking 
values. This now relies more on CSS classes provided by Material instead 
of doing finetuned logic my ourselves. This also allows showing the 
correct preview in the editor.

#287

#283

- Implement hasSchemerData in UIElement
- Fix nullable field for some elements

Better icons and reordered elements.

#267

Remove 'any's be adding proper types and using 'never'.

- problem with nullish values

Page needs to be a partial as it is only the blueprint.

Now properly finds all normal and simple drop lists.
This is achieved by a new method of unit all it's sub-components, which 
returns all contained elements with and optional type filter.

Also improve (still not very pretty) styling of the drop list props 
panel.

#260 #259

There were some cases where elements could end up without an ID.
Also just in case an error is thrown when no ID is present.

- Move methods from cloze parser helper library to cloze class.
- Cloze children are set up with a special placeholder which is then 
recognized and replaced by a new ID. And the element is recreated (like 
before) to have a proper element model class, which the TipTap editor 
doesn't provide.

- Elements initialize their fields explicitly and not with
  Object.assign, since Object.assign also assigns
  (and therefore overwrites) parent fields.
- IDService is no longer an Angular service but a basic singleton. This
  allows to use it in classes.
- The IDManager can be optionally passed to unit and element
  constructors. When present the elements check and fix their given ID
  when created.
  This way elements can be created without depending on the external
  IDManager but also with the dependency.
- ElementFactory is no longer used to create elements. Those are now
  either created directly via 'new' or via the Section, which knows the
  different element types.

Elements read as classes were overwritten by the plain objects passed to 
the constructors.

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.

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.

- Simplified the code logic. Verification is done once in the beginning 
and not on every element.
- Now uses semver to verify and compare version strings

ImportModuleVersion stores the unitDefinitionType of the loaded unit
definition. During the initialization of the unit, the individual
elements can react to the stored value.