Debugger: Combine the EngineShutdown{Ok,Failed} states

... into a EngineShutdownFinished. They were never handled differently,
and the only option is to proceed to DebuggerFinished anyway. So
simplify the state machine a bit.

Change-Id: Ied3be86fff6750abca578dc6788e4be1d895692b
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/debugger/qml/qmlengine.cpp

@@ -575,7 +575,7 @@ void QmlEngine::shutdownEngine()
    // double check (ill engine?):
     stopApplicationLauncher();
 
-    notifyEngineShutdownOk();
+    notifyEngineShutdownFinished();
     if (!isSlaveEngine())
         showMessage(QString(), StatusBar);
 }