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-tests/src/test/scala/akka/dispatch/MailboxConfigSpec.scala

@@ -39,13 +39,13 @@ abstract class MailboxSpec extends AkkaSpec with BeforeAndAfterAll with BeforeAn
 
       val exampleMessage = createMessageInvocation("test")
 
-      for (i ← 1 to config.capacity) q.enqueue(exampleMessage)
+      for (i ← 1 to config.capacity) q.enqueue(null, exampleMessage)
 
       q.numberOfMessages must be === config.capacity
       q.hasMessages must be === true
 
       intercept[MessageQueueAppendFailedException] {
-        q.enqueue(exampleMessage)
+        q.enqueue(null, exampleMessage)
       }
 
       q.dequeue must be === exampleMessage
@@ -103,7 +103,7 @@ abstract class MailboxSpec extends AkkaSpec with BeforeAndAfterAll with BeforeAn
 
     def createProducer(fromNum: Int, toNum: Int): Future[Vector[Envelope]] = spawn {
       val messages = Vector() ++ (for (i ← fromNum to toNum) yield createMessageInvocation(i))
-      for (i ← messages) q.enqueue(i)
+      for (i ← messages) q.enqueue(null, i)
       messages
     }