(which is the height of a single item)

#525

- and AnswerSchemeValue with VariableValue

- Now every element can overwrite 'getAnswerScheme' of 'UIElement'

- Method is already defined in superclass

In Player-context the constructors are to fail, when the given 
properties are not exactly as expected. Therefoore they are validated. 
When this validation fails, an environment variable is checked to 
determine if the element should be created anyway with defaults. This 
switch is off in player-context. But the Editor is not so strict and can 
create elements with incomplete parameters.

There is also a new VersionManager which checks given unit definitions 
for version-validity. Upon failing this check Player and Editor show 
error messages.
The Editor accepts a certain unit definition version, but shows a 
warning.

The Editor checks units for invalid references after loading. If any are 
found, they are removed and an info-panel is shown.

- Model constructors (elements, unit, page, section) now have a 
dedicated parameter, which makes sure all properties are set and valid. 
This way the player can instantiate models without having to interpret 
the values. Any interpretation is done only by the editor, which now is 
solely responsible for creating defaults/blueprints for new elements. 
Models can no longer be created by incomplete paramaters.

- Also refactor Stylings to have sub groups for common styles

Also restructure files containing interfaces.

- allowReplacement (moved from element to list prop)
- originList and Index are non-optional (means placeholders have to be 
used sometimes)
- Re-introduced the dict of all DropListComponents. This is the only way 
I have found to get a reference to a DropListComponent by ID. This is 
needed for returning to origin. Origin is not actually part of the 
transaction and needs to be fetched from somewhere else. Hence the dict.

- Console statements still present for debugging.

The native browser-drag-and-drop does not work in tablets. Therefore 
back to Material which works.
Features are moved over with the exception of replacing items.

- Copy-Lists now always allow putting items back.

- Renew example files
- Update versions and notes

(The array reference must not be destroyed)

HTML labels can cause errors in the Test Center for units in XML format.

Allows DropListvalueObjects which have this setting to be replaced in
lists which only allow onlyOneItem. When this switch is set, the class
sets the other 2 internal values
(originListID?: string; originListIndex) itself. Those can not be seen
nor changed from outside.

Also refactoring in lots of places around droplists. Better names, 
cleaner structure, comments.

#305

Lists with this setting delete dropped value objects. This allows 
putting items back.
(Only makes sense when used with copyOnDrop. Otherwise there can not be 
duplicate value IDs).

Also improve constructor value checks to correctly handle nullish 
booleans.

#281

... to create new references for the connectedTo array.

Duplicate value objects without keeping the reference to the old object.

- Replace Material Droplist with native HTML events
- Remove simple-drop-list element; Cloze elements now use the normal 
DropList element
- Add example units

- Fix element creation to have new value objects instead of referencing 
the old one
- Add new IDs to DropList value objects

The unit tests errored out because of circular dependencies via the 
Elementfactory class. Therefore:
- No longer creates styling/position/player-Properties. This is done in 
the Element parent class.
- Move element creation from Section to the ElementFactory class 
instead.

- remove ID handling from model classes. This is now done by the editor 
itself (mainly unit service).
- Rename and move IDManager to editor as IDService

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

- 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.