feat: add CanClose hook to easily block window closing.

demo/Ursa.Demo/Views/MainWindow.axaml.cs

@@ -1,3 +1,4 @@
+using System.Threading.Tasks;
 using Ursa.Controls;
 
 namespace Ursa.Demo.Views;
@@ -8,4 +9,10 @@ public partial class MainWindow : UrsaWindow
     {
         InitializeComponent();
     }
+
+    protected override async Task<bool> CanClose()
+    {
+        var result = await MessageBox.ShowOverlayAsync("Are you sure you want to exit?", "Exit", button: MessageBoxButton.YesNo);
+        return result == MessageBoxResult.Yes;
+    }
 }