improve DeadLetter reporting

(since I know now what’s causing these Jenkins failures ;-) )

- include recipient in DeadLetter
- include recipient in calls to enqueue/systemEnqueue
- move DeadLetterMailbox to ActorSystem (saves some space, too)
- hook up DeadLetterMailbox so it sends DeadLetters to app.deadLetters,
  which publishes them on the eventStream
- subscribe TestEventListener to DeadLetter and turn it into Warning

The generated warnings about ChildTerminated are very much correct, they
remind us that we still need to fix supervisor.stop() to await all
children’s death before actually committing suicide.

akka-actor/src/main/scala/akka/dispatch/Dispatcher.scala

@@ -78,13 +78,13 @@ class Dispatcher(
 
   protected[akka] def dispatch(receiver: ActorCell, invocation: Envelope) = {
     val mbox = receiver.mailbox
-    mbox enqueue invocation
+    mbox.enqueue(receiver.self, invocation)
     registerForExecution(mbox, true, false)
   }
 
   protected[akka] def systemDispatch(receiver: ActorCell, invocation: SystemMessage) = {
     val mbox = receiver.mailbox
-    mbox systemEnqueue invocation
+    mbox.systemEnqueue(receiver.self, invocation)
     registerForExecution(mbox, false, true)
   }