Review property visibility for all classes.

[ghost]

This will need to be checked because during development this was not sufficiently taken into account (and I personally feel still insecure how to use visibility correctly).

[ghost]

Visibility is set to private everywhere except for $u_action which needs to be public. This could still be subject of a refactoring, but this would then better happen in a new issue/pr. Closing as no longer needed for the initial release.