Setup Wizard restructuring

- Remove polymorphic base, this only existed because TKMM has a desktop/switch setup prodecure difference and has 2 implementations of the setup wizard. We only need one.
- Remove Systems/UI file split, they're all in Ryujinx.Ava.UI now
- made NotificationHelper instance-based to allow you to encapsulate notifications to a window that magically disappear when the window is closed, instead of switching to showing on the main window.

src/Ryujinx/UI/Windows/MainWindow.axaml.cs

@@ -15,7 +15,6 @@ using Ryujinx.Ava.Systems;
 using Ryujinx.Ava.Systems.AppLibrary;
 using Ryujinx.Ava.Systems.Configuration;
 using Ryujinx.Ava.Systems.Configuration.UI;
-using Ryujinx.Ava.Systems.SetupWizard;
 using Ryujinx.Ava.UI.Applet;
 using Ryujinx.Ava.UI.Helpers;
 using Ryujinx.Ava.UI.Models;
@@ -136,7 +135,7 @@ namespace Ryujinx.Ava.UI.Windows
         {
             base.OnApplyTemplate(e);
 
-            NotificationHelper.SetNotificationManager(this);
+            NotificationHelper.Shared = new NotificationHelper(this);
 
             Executor.ExecuteBackgroundAsync(async () =>
             {