ISelectionModel implements INotifyPropertyChanged.

This will be needed for monitoring the `AnchorIndex` in order to auto-scroll.

src/Avalonia.Controls/ISelectionModel.cs

@@ -5,10 +5,11 @@
 
 using System;
 using System.Collections.Generic;
+using System.ComponentModel;
 
 namespace Avalonia.Controls
 {
-    public interface ISelectionModel
+    public interface ISelectionModel : INotifyPropertyChanged
     {
         IndexPath AnchorIndex { get; set; }
         IndexPath SelectedIndex { get; set; }

src/Avalonia.Controls/SelectionModel.cs

@@ -12,7 +12,7 @@ using Avalonia.Controls.Utils;
 
 namespace Avalonia.Controls
 {
-    public class SelectionModel : ISelectionModel, INotifyPropertyChanged, IDisposable
+    public class SelectionModel : ISelectionModel, IDisposable
     {
         private readonly SelectionNode _rootNode;
         private bool _singleSelect;