feat: add overlay dialog sample.

demo/Ursa.Demo/Dialogs/PlainDialog.axaml.cs

@@ -0,0 +1,13 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+
+namespace Ursa.Demo.Dialogs;
+
+public partial class PlainDialog : UserControl
+{
+    public PlainDialog()
+    {
+        InitializeComponent();
+    }
+}