Keep the GUI alive when closing a game (#888)

* Keep the GUI alive when closing a game

Make HLE.Switch init when starting a game and dispose it when closing
the GlScreen.

This also make HLE in charge of disposing the audio and gpu backend.

* Address Ac_k's comments

* Make sure to dispose the Discord module and use GTK quit method

Also update Discord Precense when closing a game.

* Make sure to dispose MainWindow

* Address gdk's comments

Ryujinx/Program.cs

@@ -43,7 +43,6 @@ namespace Ryujinx
                 ConfigurationState.Instance.ToFileFormat().SaveConfig(configurationPath);
             }
 
-
             Profile.Initialize();
 
             Application.Init();