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.HLE/HOS/Services/Am/AppletOE/ApplicationProxyService/ApplicationProxy/IApplicationFunctions.cs

@@ -60,7 +60,7 @@ namespace Ryujinx.HLE.HOS.Services.Am.AppletOE.ApplicationProxyService.Applicati
                     "No control file was found for this game. Using a dummy one instead. This may cause inaccuracies in some games.");
             }
 
-            Result result = EnsureApplicationSaveData(context.Device.System.FsClient, out long requiredSize, titleId,
+            Result result = EnsureApplicationSaveData(context.Device.FileSystem.FsClient, out long requiredSize, titleId,
                 ref context.Device.System.ControlData.Value, ref userId);
 
             context.ResponseData.Write(requiredSize);