MainWindow: shutdown different input interfaces

`MainWindow` initializes a number of input interfaces but never shuts
them down. This was causing a crash-after-exit on macOS where the
ControllerInterface backend stores a `std::thread` object in a static
variable and only stops it when ControllerInterface::Shutdown is called.

Source/Core/DolphinQt2/MainWindow.h

@@ -66,6 +66,7 @@ private:
   void ConnectPathsDialog();
 
   void InitControllers();
+  void ShutdownControllers();
 
   void StartGame(const QString& path);
   void ShowRenderWidget();