feat: add UserControl, add Window Resources, update folder structure.

src/Semi.Demo/App.axaml.cs

@@ -0,0 +1,24 @@
+using Avalonia;
+using Avalonia.Controls.ApplicationLifetimes;
+using Avalonia.Markup.Xaml;
+
+namespace Semi.Demo
+{
+    public partial class App : Application
+    {
+        public override void Initialize()
+        {
+            AvaloniaXamlLoader.Load(this);
+        }
+
+        public override void OnFrameworkInitializationCompleted()
+        {
+            if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
+            {
+                desktop.MainWindow = new MainWindow();
+            }
+
+            base.OnFrameworkInitializationCompleted();
+        }
+    }
+}