Ensure containers are ordered correctly.

Fixes #1936.

src/Avalonia.Controls/Generators/ItemContainerGenerator.cs

@@ -15,7 +15,7 @@ namespace Avalonia.Controls.Generators
     /// </summary>
     public class ItemContainerGenerator : IItemContainerGenerator
     {
-        private Dictionary<int, ItemContainerInfo> _containers = new Dictionary<int, ItemContainerInfo>();
+        private SortedDictionary<int, ItemContainerInfo> _containers = new SortedDictionary<int, ItemContainerInfo>();
 
         /// <summary>
         /// Initializes a new instance of the <see cref="ItemContainerGenerator"/> class.
@@ -246,4 +246,4 @@ namespace Avalonia.Controls.Generators
             Recycled?.Invoke(this, e);
         }
     }
-}
+}