Added missing Materialized event invocation.

In TreeContainerIndex. Also add TODO concerning ItemContainerEventArgs.

src/Avalonia.Controls/Generators/ItemContainerEventArgs.cs

@@ -27,6 +27,10 @@ namespace Avalonia.Controls.Generators
         /// </summary>
         /// <param name="startingIndex">The index of the first container in the source items.</param>
         /// <param name="containers">The containers.</param>
+        /// <remarks>
+        /// TODO: Do we really need to pass in StartingIndex here? The ItemContainerInfo objects
+        /// have an index, and what happens if the contains passed in aren't sequential?
+        /// </remarks>
         public ItemContainerEventArgs(
             int startingIndex, 
             IList<ItemContainerInfo> containers)

src/Avalonia.Controls/Generators/TreeContainerIndex.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Linq;
 
 namespace Avalonia.Controls.Generators
 {
@@ -68,8 +69,9 @@ namespace Avalonia.Controls.Generators
         /// <summary>
         /// Removes a set of containers from the index.
         /// </summary>
+        /// <param name="startingIndex">The index of the first item.</param>
         /// <param name="containers">The item containers.</param>
-        public void Remove(IEnumerable<ItemContainerInfo> containers)
+        public void Remove(int startingIndex, IEnumerable<ItemContainerInfo> containers)
         {
             foreach (var container in containers)
             {
@@ -77,6 +79,10 @@ namespace Avalonia.Controls.Generators
                 _containerToItem.Remove(container.ContainerControl);
                 _itemToContainer.Remove(item);
             }
+
+            Dematerialized?.Invoke(
+                this,
+                new ItemContainerEventArgs(startingIndex, containers.ToList()));
         }
 
         /// <summary>

src/Avalonia.Controls/Generators/TreeItemContainerGenerator.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using Avalonia.Controls.Templates;
 using Avalonia.Data;
 
@@ -102,19 +103,19 @@ namespace Avalonia.Controls.Generators
         public override IEnumerable<ItemContainerInfo> Clear()
         {
             var items = base.Clear();
-            Index.Remove(items);
+            Index.Remove(0, items);
             return items;
         }
 
         public override IEnumerable<ItemContainerInfo> Dematerialize(int startingIndex, int count)
         {
-            Index.Remove(GetContainerRange(startingIndex, count));
+            Index.Remove(startingIndex, GetContainerRange(startingIndex, count));
             return base.Dematerialize(startingIndex, count);
         }
 
         public override IEnumerable<ItemContainerInfo> RemoveRange(int startingIndex, int count)
         {
-            Index.Remove(GetContainerRange(startingIndex, count));
+            Index.Remove(startingIndex, GetContainerRange(startingIndex, count));
             return base.RemoveRange(startingIndex, count);
         }