Add GCNotifier class.

src/Avalonia.Base/Utilities/GCNotifier.cs

@@ -0,0 +1,29 @@
+// Copyright (c) The Avalonia Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
+using System;
+
+namespace Avalonia.Utilities
+{
+    public class GCNotifier
+    {
+        public static event EventHandler GarbageCollected;
+
+        static GCNotifier()
+        {
+            new GCNotifier();
+        }
+
+        ~GCNotifier()
+        {
+            if (Environment.HasShutdownStarted || AppDomain.CurrentDomain.IsFinalizingForUnload())
+            {
+                return;
+            }
+
+            new GCNotifier();
+
+            GarbageCollected?.Invoke(null, EventArgs.Empty);
+        }
+    }
+}