Added common file dialog service; upgraded to Avalonia 11.3.7

Source/CommonDialogs/IDirectoryPickerService.cs

@@ -0,0 +1,36 @@
+#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 System.Threading.Tasks;
+
+using Avalonia.Platform.Storage;
+
+namespace Nuclex.Avalonia.CommonDialogs {
+
+  /// <summary>Service that asks the user to pick a directory</summary>
+  public interface IDirectoryPickerService {
+
+    /// <summary>Asks the user to select a directory</summary>
+    /// <returns>A task that will provide the directory the user has selected</returns>
+    Task<IStorageFolder?> AskForDirectory(string caption);
+
+  }
+
+} // namespace Nuclex.Avalonia.CommonDialogs