Qt: Fix emulation stopped signal

Same old bug as in WX. The core is not shut down until the on stopped
callback is invoked.

Source/Core/DolphinQt2/MainWindow.h

@@ -69,6 +69,8 @@ private:
   void InitControllers();
   void ShutdownControllers();
 
+  void InitCoreCallbacks();
+
   void StartGame(const QString& path);
   void ShowRenderWidget();
   void HideRenderWidget();