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/FileSystem/StorageId.cs

@@ -1,6 +1,6 @@
 namespace Ryujinx.HLE.FileSystem
 {
-    internal enum StorageId
+    public enum StorageId
     {
         None,
         Host,