Continue working on event API

[darkwisebear]

This PR is far from perfect but allows for some comments already. The main points to be fixed:

• There is no generic solution yet how to turn an interface into an instance.
• There is a violation of the orphan rules. This doesn't really show here as the sample code and the API are in the same crate. We need to separate those entities into
  • API crate
  • Sample runtime crate
  • Sample user code crate

[darkwisebear]

Included in #8.