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/SetupWizardPage.cs

@@ -0,0 +1,59 @@
+using Avalonia.Controls.Presenters;
+using CommunityToolkit.Mvvm.ComponentModel;
+using CommunityToolkit.Mvvm.Input;
+using Ryujinx.Ava.Common.Locale;
+using Ryujinx.Ava.UI.ViewModels;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Ryujinx.Ava.UI.SetupWizard
+{
+    public partial class SetupWizardPage(ContentPresenter contentPresenter, RyujinxSetupWizard ownerWizard, bool isFirstPage = false) : BaseModel
+    {
+        private bool? _result;
+        private readonly CancellationTokenSource _cts = new();
+
+        public bool IsFirstPage { get; } = isFirstPage;
+
+        [ObservableProperty]
+        public partial string? Title { get; set; }
+
+        [ObservableProperty]
+        public partial object? Content { get; set; }
+
+        [ObservableProperty] public partial object? HelpContent { get; set; }
+
+        [ObservableProperty]
+        public partial object? ActionContent { get; set; } = LocaleManager.Instance[LocaleKeys.SetupWizardActionNext];
+
+        [RelayCommand]
+        private void MoveBack()
+        {
+            _result = false;
+            _cts.Cancel();
+        }
+
+        [RelayCommand]
+        private void MoveNext()
+        {
+            _result = true;
+            _cts.Cancel();
+        }
+
+        public async ValueTask<bool> Show()
+        {
+            contentPresenter.Content = new SetupWizardPageView { ViewModel = this };
+
+            try
+            {
+                await Task.Delay(-1, _cts.Token);
+            }
+            catch (TaskCanceledException)
+            {
+                return _result ?? false;
+            }
+
+            return false;
+        }
+    }
+}