add scafolding for native osx menu

src/Avalonia.Native/AvaloniaNativeMenuExporter.cs

@@ -0,0 +1,25 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using Avalonia.Controls;
+using Avalonia.Controls.Platform;
+
+namespace Avalonia.Native
+{
+    class AvaloniaNativeMenuExporter : ITopLevelNativeMenuExporter
+    {
+        public bool IsNativeMenuExported => throw new NotImplementedException();
+
+        public event EventHandler OnIsNativeMenuExportedChanged;
+
+        public void SetNativeMenu(NativeMenu menu)
+        {
+            throw new NotImplementedException();
+        }
+
+        public void SetPrependApplicationMenu(bool prepend)
+        {
+            throw new NotImplementedException();
+        }
+    }
+}

src/Avalonia.Native/WindowImpl.cs

@@ -3,13 +3,14 @@
 
 using System;
 using Avalonia.Controls;
+using Avalonia.Controls.Platform;
 using Avalonia.Native.Interop;
 using Avalonia.Platform;
 using Avalonia.Platform.Interop;
 
 namespace Avalonia.Native
 {
-    public class WindowImpl : WindowBaseImpl, IWindowImpl
+    public class WindowImpl : WindowBaseImpl, IWindowImpl, ITopLevelImplWithNativeMenuExporter
     {
         private readonly IAvaloniaNativeFactory _factory;
         private readonly AvaloniaNativePlatformOptions _opts;
@@ -22,6 +23,8 @@ namespace Avalonia.Native
             {
                 Init(_native = factory.CreateWindow(e), factory.CreateScreens());
             }
+
+            NativeMenuExporter = new AvaloniaNativeMenuExporter();
         }
 
         class WindowEvents : WindowBaseEvents, IAvnWindowEvents
@@ -104,6 +107,9 @@ namespace Avalonia.Native
         }
 
         public Func<bool> Closing { get; set; }
+
+        public ITopLevelNativeMenuExporter NativeMenuExporter { get; }
+
         public void Move(PixelPoint point) => Position = point;
 
         public override IPopupImpl CreatePopup() =>