- 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.