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

- 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.

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.

- Split text field and drop list, so they can have differing props
- Improve their dimension default values
- Improve alignment of all cloze children components
- Add dynamic width parameter of toggle buttons to dimension properties
panel. Therefore the postion tab can always be shown and no longer needs
a condition.

This reverts commit d053d03c.

This feature will first be developed properly in a branch.

Cloze documents now use this correctly.

Change import paths of CommonModule files in editor and player

Instead, use Textfied component with the property isClozeChild

- Units and element are now built via the factory. This ensures only the
  wanted properties are present. All others are discarded.
- Since the sanatizer is supposed to check the unit defintion version
  it is used by player and editor and therefore put in common.
  This in turn makes it necessary to put the JSON resolver settings
  in the root-TSConfig file.
- Renamed a couple of properties to have clearer names.

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.