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.

#401

- 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

- Add translations for operators
- Improve and extend the evaluation of visibility rules

- Refactor directive

- Basic implementation

Also restructure files containing interfaces.

- A suitable version was not available after update of Angular
- Reactivate test for valid pages

Most importantly remove the standalone switch and let the service always 
send postMessage, even when there is no surrounding application 
listening. This helps testability and has no downsides.

Also:
- Make methods static where applicable
- Remove useless variables and methods
- Improve variable initialisation (to allow undefined)

- This allows to assign a font to a text in MatFabButtons

Element margins and grid sizes now have a measure unit attached. The 
translation logic to merge unit and value are done via Pipe.