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/SetupWizard/SetupWizardPageContext.cs

@@ -0,0 +1,13 @@
+using Gommon;
+using Ryujinx.Ava.UI.Helpers;
+using Ryujinx.Ava.UI.ViewModels;
+
+namespace Ryujinx.Ava.UI.SetupWizard
+{
+    public abstract class SetupWizardPageContext : BaseModel
+    {
+        public NotificationHelper Notifications { get; init; }
+
+        public abstract Result CompleteStep();
+    }
+}