Fix license headers

src/Avalonia.Controls/DockPanel.cs

@@ -1,7 +1,12 @@
+// This source file is adapted from the Windows Presentation Foundation project. 
+// (https://github.com/dotnet/wpf/) 
+// 
+// Licensed to The Avalonia Project under MIT License, courtesy of The .NET Foundation.
+
+using System;
+
 namespace Avalonia.Controls
 {
-    using System;
-
     /// <summary>
     /// Defines the available docking modes for a control in a <see cref="DockPanel"/>.
     /// </summary>

src/Avalonia.Controls/StackPanel.cs

@@ -1,5 +1,7 @@
-// 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.
+// This source file is adapted from the Windows Presentation Foundation project. 
+// (https://github.com/dotnet/wpf/) 
+// 
+// Licensed to The Avalonia Project under MIT License, courtesy of The .NET Foundation.
 
 using System;
 using Avalonia.Input;

src/Avalonia.Controls/WrapPanel.cs

@@ -1,9 +1,7 @@
-// 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.Collections.Generic;
-using System.Diagnostics;
-using System.Linq;
+// This source file is adapted from the Windows Presentation Foundation project. 
+// (https://github.com/dotnet/wpf/) 
+// 
+// Licensed to The Avalonia Project under MIT License, courtesy of The .NET Foundation.
 
 using Avalonia.Input;
 using Avalonia.Utilities;