Use ItemsPresenter in DropDown template.

Breaks a few things  - they need properly fixing.

Perspex.Controls/DropDown.cs

@@ -7,8 +7,7 @@
 namespace Perspex.Controls
 {
     using System;
-    using Perspex.Collections;
-    using Perspex.Controls.Presenters;
+    using Generators;
     using Perspex.Controls.Primitives;
     using Perspex.Controls.Templates;
     using Perspex.Input;
@@ -58,6 +57,11 @@ namespace Perspex.Controls
             set { this.SetValue(IsDropDownOpenProperty, value); }
         }
 
+        protected override IItemContainerGenerator CreateItemContainerGenerator()
+        {
+            return new ItemContainerGenerator<ListBoxItem>(this);
+        }
+
         protected override void OnPointerPressed(PointerPressEventArgs e)
         {
             if (!this.IsDropDownOpen)

Perspex.Controls/Presenters/ItemsPresenter.cs

@@ -46,6 +46,13 @@ namespace Perspex.Controls.Presenters
             ItemsProperty.Changed.AddClassHandler<ItemsPresenter>(x => x.ItemsChanged);
         }
 
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ItemsPresenter"/> class.
+        /// </summary>
+        public ItemsPresenter()
+        {
+        }
+
         /// <summary>
         /// Gets the <see cref="IItemContainerGenerator"/> used to generate item container
         /// controls.

Perspex.Themes.Default/DropDownStyle.cs

@@ -9,6 +9,7 @@ namespace Perspex.Themes.Default
     using System.Linq;
     using System.Reactive.Linq;
     using Perspex.Controls;
+    using Controls.Presenters;
     using Perspex.Controls.Primitives;
     using Perspex.Controls.Shapes;
     using Perspex.Controls.Templates;
@@ -91,10 +92,15 @@ namespace Perspex.Themes.Default
                         },
                         new Popup
                         {
-                            Child = new ListBox
+                            Child = new Border
                             {
-                                [~ListBox.ItemsProperty] = control[~DropDown.ItemsProperty],
-                                [~~ListBox.SelectedItemProperty] = control[~~DropDown.SelectedItemProperty],
+                                BorderBrush = Brushes.Black,
+                                BorderThickness = 1,
+                                Padding = new Thickness(4),
+                                Child = new ItemsPresenter
+                                {
+                                    [~ListBox.ItemsProperty] = control[~DropDown.ItemsProperty],
+                                }
                             },
                             PlacementTarget = control,
                             StaysOpen = false,