Qt: Restore fullscreen resolution when regaining focus

Source/Core/DolphinQt2/RenderWidget.h

@@ -28,6 +28,7 @@ signals:
   void HandleChanged(void* handle);
   void StateChanged(bool fullscreen);
   void SizeChanged(int new_width, int new_height);
+  void FocusChanged(bool focus);
 
 private:
   void HandleCursorTimer();