feat: add new event.

src/Ursa/Controls/NavMenu/SelectionChangingEventArgs.cs

@@ -0,0 +1,24 @@
+using System.Collections;
+using Avalonia.Interactivity;
+
+namespace Ursa.Controls;
+
+public class SelectionChangingEventArgs: RoutedEventArgs
+{
+    /// <summary>Gets the items that were added to the selection.</summary>
+    public IList AddedItems { get; }
+
+    /// <summary>Gets the items that were removed from the selection.</summary>
+    public IList RemovedItems { get; }
+    
+    /// <summary>
+    /// Gets or sets a value indicating whether the selection can be changed. If set to <c>false</c>, the selection will not change.
+    /// </summary>
+    public bool CanSelect { get; set; } = true;
+    
+    public SelectionChangingEventArgs(RoutedEvent routedEvent, IList removedItems, IList addedItems): base(routedEvent)
+    {
+        RemovedItems = removedItems;
+        AddedItems = addedItems;
+    }
+}