feat: initialize, setup demo. make sure MVVM works.

demo/Ursa.Demo/DataTemplates/MenuDataTemplateSelector.cs

@@ -0,0 +1,26 @@
+using Avalonia.Controls;
+using Avalonia.Controls.Templates;
+using Ursa.Demo.ViewModels;
+
+namespace Ursa.Demo.Converters;
+
+public class MenuDataTemplateSelector: IDataTemplate
+{
+    public IDataTemplate? MenuTemplate { get; set; }
+    public IDataTemplate? SeparatorTemplate { get; set; }
+    
+    public Control? Build(object? param)
+    {
+        if (param is MenuItemViewModel vm)
+        {
+            if (vm.IsSeparator) return SeparatorTemplate?.Build(vm);
+            else return MenuTemplate?.Build(vm);
+        }
+        return null;
+    }
+
+    public bool Match(object? data)
+    {
+        return true;
+    }
+}