- Method is already defined in superclass

#452

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.

Measure holds the value and a unit for CSS properties.

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

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

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.

#283

- 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.

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.