Initial porting attempt of my WinForms MVVM library to Avalonia; added IWindowScope as a new reature because this library should support Microsoft's dependency injector

Source/NullActiveWindowTracker.cs

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+#region Apache License 2.0
+/*
+Nuclex Foundation libraries for .NET
+Copyright (C) 2002-2025 Markus Ewald / Nuclex Development Labs
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+#endregion // Apache License 2.0
+
+using System;
+
+using Avalonia.Controls;
+
+namespace Nuclex.Avalonia {
+
+  /// <summary>Dummy implementation of the active window tracker service</summary>
+  internal class NullActiveWindowTracker : IActiveWindowTracker {
+
+    /// <summary>The default instance of the dummy window tracker</summary>
+    public static readonly NullActiveWindowTracker Default = new NullActiveWindowTracker();
+
+    /// <summary>The currently active top-level or modal window</summary>
+    public Window? ActiveWindow { get { return null; } }
+
+  }
+
+} // namespace Nuclex.Avalonia