Better icons and reordered elements.

#267

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

- problem with nullish values

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.

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.