QmlDebug: Parse application output to track debugees state

Setting up the debugger connection might fail for a couple of reasons.
Check the application output to show the user a more detailed error
message.

Reviewed-by: hjk

src/plugins/debugger/qml/qmlcppengine.h

@@ -91,6 +91,9 @@ public:
     void handleRemoteSetupDone(int gdbServerPort, int qmlPort);
     void handleRemoteSetupFailed(const QString &message);
 
+    void showMessage(const QString &msg, int channel = LogDebug,
+        int timeout = -1) const;
+
 protected:
     void detachDebugger();
     void executeStep();