This is needed in on drop logic to set the new value.

The dragged image did contain carry information about it's element.
Also remove useless drag and drop functionality for the placeholder.

This reverts commit da09ac1e.

- 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

#264

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.

The service is no longer an Angular service, but is now a basic 
singleton.

The creation of a temporary cloze elements fails when it encounters 
unknown child types.
Therefore we use a static ClozeElement method to get the child elements 
from the document instead of the whole element.

- 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.

Add 2 new menu buttons to the section menu.
One simply copies the selected section to the clipboard.
The other opens a dialog where a copied section can be pasted. The 
section element's IDs are checked if they are available and a warning is 
shown.

- 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.

- no event for elements
- only one event für presented pages

- Fix related tests

- Now only UIElements are found
- HTML parts of the text editor are ignored

- FloatingMarkingBarComponent no longer has the elementModel property

- Error was raised when loading a second unit. It was caused by the
binding of the UnitStateService to the UnitMenuComponent