Checks if the correct Play background runner is available and sends info...

... to client if this is the case.

Contains some refactoring of ``SbtClientActor`` that is not directly related to the implemented functionality - more a function of the boy scout rule.

Lines 142-147 in ``SbtClientLifeCycleHandlerActor`` are the ones checking if the Play background runner is available.
This is an interesting change.  Is the default strategy `SupervisorStrategy.stoppingStrategy`?