Add ContainerType property to IItemContainerGenerator

src/Avalonia.Controls/Generators/IItemContainerGenerator.cs

@@ -22,6 +22,11 @@ namespace Avalonia.Controls.Generators
         /// </summary>
         IDataTemplate ItemTemplate { get; set; }
 
+        /// <summary>
+        /// Gets the ContainerType, or null if its an untyped ContainerGenerator.
+        /// </summary>
+        Type ContainerType { get; }
+
         /// <summary>
         /// Signalled whenever new containers are materialized.
         /// </summary>