Table children do not have some properties, like position.

This fixes Tables which houses elements without dimensions.

Move element specific components to sub folder and just generally 
improve the file/component structure.

#552

#618

#608

- Prevent input of 'Dead' keys

- and AnswerSchemeValue with VariableValue

- Remove unused imports

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

- Method is already defined in superclass

#583

Small cells and canBeCrossedOut are property of the row as they are all 
cells or none.
Additional property 'is2DigitHelperRow' is for different types of helper 
rows: single-digit in no crossing out or 2 digits with crossing out. 
2-digit is only for the top 2 substraction rows.

#589

This holds variable names, taken from the GeoGebra applet, and uses 
those variable values in the value. This makes result analysis easier.

#461
- This property will be used for all input assistance presets
  in the future

#506

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

- Only elements with this property are considered in the player when
determining the presentation state
- Add the registration of CompoundElements at the UnitStateservice
- Fix visibility handling of section with simple rules

- Remove dynamicPositioning from PositionProperties. This is not 
knowledge that the element needs to know. In the few places it was used, 
it was fixed or solved by asking the containing section about which 
positioning is active.

- Fix and improve cloze child sizing. All can now have dynamic width and 
height.

- Fix empty lines in ClozeDocument rendering

- Editor SelectionService now knows if a cloze child is selected. This 
way the PropertiesPanel can show valid dimension inputs regardless of 
the underlying section.