Return InstancedBinding from TreeDataTemplate.

This hopefully fixes the part of issue #284 related to TreeDataTemplate.
MemberSelector needs a similar fix.

src/Markup/Perspex.Markup.Xaml/Templates/TreeDataTemplate.cs

@@ -36,12 +36,12 @@ namespace Perspex.Markup.Xaml.Templates
             }
         }
 
-        public IEnumerable ItemsSelector(object item)
+        public InstancedBinding ItemsSelector(object item)
         {
             if (ItemsSource != null)
             {
                 var obs = new ExpressionObserver(item, ItemsSource.Path);
-                return obs.Take(1).Wait() as IEnumerable;
+                return new InstancedBinding(obs);
             }
 
             return null;

src/Perspex.Controls/Generators/TreeItemContainerGenerator.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using Perspex.Controls.Templates;
+using Perspex.Data;
 
 namespace Perspex.Controls.Generators
 {
@@ -76,7 +77,7 @@ namespace Perspex.Controls.Generators
                 var result = new T();
 
                 result.SetValue(ContentProperty, template.Build(item));
-                result.SetValue(ItemsProperty, template.ItemsSelector(item));
+                BindingOperations.Apply(result, ItemsProperty, template.ItemsSelector(item), null);
 
                 if (!(item is IControl))
                 {

src/Perspex.Controls/Templates/FuncTreeDataTemplate.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections;
 using System.Reflection;
+using Perspex.Data;
 
 namespace Perspex.Controls.Templates
 {
@@ -59,9 +60,9 @@ namespace Perspex.Controls.Templates
         /// </summary>
         /// <param name="item">The item.</param>
         /// <returns>The child items, or null if no child items.</returns>
-        public IEnumerable ItemsSelector(object item)
+        public InstancedBinding ItemsSelector(object item)
         {
-            return this?._itemsSelector(item);
+            return new InstancedBinding(this?._itemsSelector(item));
         }
 
         /// <summary>

src/Perspex.Controls/Templates/ITreeDataTemplate.cs

@@ -1,7 +1,7 @@
 // Copyright (c) The Perspex Project. All rights reserved.
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
-using System.Collections;
+using Perspex.Data;
 
 namespace Perspex.Controls.Templates
 {
@@ -14,7 +14,10 @@ namespace Perspex.Controls.Templates
         /// Selects the child items of an item.
         /// </summary>
         /// <param name="item">The item.</param>
-        /// <returns>The child items, or null if no child items.</returns>
-        IEnumerable ItemsSelector(object item);
+        /// <returns>
+        /// An <see cref="InstancedBinding"/> holding the items, or an observable that tracks the
+        /// items. May return null if no child items.
+        /// </returns>
+        InstancedBinding ItemsSelector(object item);
     }
 }