API super-class

[Bethibande]

Task

Add a super-class (i.e interface or abstract class) for the Api classes generated by the processor.
This class should contain boilerplate code like the close function or another interface for the localActorSystem function of the companion object.
The super-class should also delegate some functions of the wrapped AbstractActor instance like isClosed and isRunning,
the send function may also become part of the super-class.

Motivation

This will reduce the boilerplate generated by the processor. Chaning this boilerplate code or adding functionallity will be easier and may not even require a change to the code-generation after this change.
Implementing this change will also make implementing custom actors without the processor easier.

[Bethibande]

Close command has not been abstracted yet as the way close commands are handled will be changed with #22.