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/SetupWizardPageView.axaml.cs

@@ -0,0 +1,13 @@
+using Ryujinx.Ava.UI.Controls;
+
+namespace Ryujinx.Ava.UI.SetupWizard
+{
+    public partial class SetupWizardPageView : RyujinxControl<SetupWizardPage>
+    {
+        public SetupWizardPageView()
+        {
+            InitializeComponent();
+        }
+    }
+}
+