Fix AlwaysSelected selected state.

Fixes #2901. Two things needed to be done here:

- When an item is removed, causing indexes to be reassigned, raise `Recycled` so that `SelectingItemsControl` knows to update the selection state
- Update selection state in `SelectingItemsControl` when the selected item changes, but the selected index does not (due to an item being remove)

src/Avalonia.Controls/Generators/ItemContainerGenerator.cs

@@ -128,6 +128,12 @@ namespace Avalonia.Controls.Generators
                 }
 
                 Dematerialized?.Invoke(this, new ItemContainerEventArgs(startingIndex, result));
+
+                if (toMove.Count > 0)
+                {
+                    var containers = toMove.Select(x => x.Value).ToList();
+                    Recycled?.Invoke(this, new ItemContainerEventArgs(containers[0].Index, containers));
+                }
             }
 
             return result;

src/Avalonia.Controls/Primitives/SelectingItemsControl.cs

@@ -979,13 +979,14 @@ namespace Avalonia.Controls.Primitives
             }
 
             var item = ElementAt(Items, index);
+            var itemChanged = !Equals(item, oldItem);
             var added = -1;
             HashSet<int> removed = null;
 
             _selectedIndex = index;
             _selectedItem = item;
 
-            if (oldIndex != index || _selection.HasMultiple)
+            if (oldIndex != index || itemChanged || _selection.HasMultiple)
             {
                 if (clear)
                 {
@@ -1022,7 +1023,7 @@ namespace Avalonia.Controls.Primitives
                     index);
             }
 
-            if (!Equals(item, oldItem))
+            if (itemChanged)
             {
                 RaisePropertyChanged(
                     SelectedItemProperty,