Move SynchronizeItems to TreeView.

As it's no longer used in `SelectingItemsControl`.

src/Avalonia.Controls/Primitives/SelectingItemsControl.cs

@@ -648,28 +648,6 @@ namespace Avalonia.Controls.Primitives
             return false;
         }
 
-        /// <summary>
-        /// Makes a list of objects equal another (though doesn't preserve order).
-        /// </summary>
-        /// <param name="items">The items collection.</param>
-        /// <param name="desired">The desired items.</param>
-        internal static void SynchronizeItems(IList items, IEnumerable<object> desired)
-        {
-            var list = items.Cast<object>().ToList();
-            var toRemove = list.Except(desired).ToList();
-            var toAdd = desired.Except(list).ToList();
-
-            foreach(var i in toRemove)
-            {
-                items.Remove(i);
-            }
-
-            foreach (var i in toAdd)
-            {
-                items.Add(i);
-            }
-        }
-
         /// <summary>
         /// Gets a range of items from an IEnumerable.
         /// </summary>

src/Avalonia.Controls/TreeView.cs

@@ -409,7 +409,7 @@ namespace Avalonia.Controls
 
                 if (this.SelectionMode == SelectionMode.Multiple && Match(keymap.SelectAll))
                 {
-                    SelectingItemsControl.SynchronizeItems(SelectedItems, ItemContainerGenerator.Index.Items);
+                    SynchronizeItems(SelectedItems, ItemContainerGenerator.Index.Items);
                     e.Handled = true;
                 }
             }
@@ -521,7 +521,7 @@ namespace Avalonia.Controls
             }
             else if (multi && range)
             {
-                SelectingItemsControl.SynchronizeItems(
+                SynchronizeItems(
                     SelectedItems,
                     GetItemsInRange(selectedContainer as TreeViewItem, container as TreeViewItem));
             }
@@ -778,5 +778,27 @@ namespace Avalonia.Controls
                 container.Classes.Set(":selected", selected);
             }
         }
+
+        /// <summary>
+        /// Makes a list of objects equal another (though doesn't preserve order).
+        /// </summary>
+        /// <param name="items">The items collection.</param>
+        /// <param name="desired">The desired items.</param>
+        private static void SynchronizeItems(IList items, IEnumerable<object> desired)
+        {
+            var list = items.Cast<object>().ToList();
+            var toRemove = list.Except(desired).ToList();
+            var toAdd = desired.Except(list).ToList();
+
+            foreach (var i in toRemove)
+            {
+                items.Remove(i);
+            }
+
+            foreach (var i in toAdd)
+            {
+                items.Add(i);
+            }
+        }
     }
 }