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.Graphics.Gpu/GpuContext.cs

@@ -116,6 +116,7 @@ namespace Ryujinx.Graphics.Gpu
             Methods.ShaderCache.Dispose();
             Methods.BufferManager.Dispose();
             Methods.TextureManager.Dispose();
+            Renderer.Dispose();
         }
     }
 }