Add a method to allow mac titlebar colors to be set.

src/Avalonia.Native/WindowImpl.cs

@@ -51,6 +51,11 @@ namespace Avalonia.Native
             _native.HasDecorations = enabled;
         }
 
+        public void SetTitleBarColor (Avalonia.Media.Color color)
+        {
+            _native.SetTitleBarColor(new AvnColor { Alpha = color.A, Red = color.R, Green = color.G, Blue = color.B });
+        }
+
         public void SetTitle(string title)
         {
         }