Don't explicitly set ItemsControl panel orientation.

That way, can be overridden in styles and thus doesn't need to be
overridden in TabStrip.

Perspex.Controls/ItemsControl.cs

@@ -23,7 +23,7 @@ namespace Perspex.Controls
     {
         [SuppressMessage("Microsoft.StyleCop.CSharp.NamingRules", "SA1202:ElementsMustBeOrderedByAccess", Justification = "Needs to be before or a NullReferenceException is thrown.")]
         private static readonly ItemsPanelTemplate DefaultPanel =
-            new ItemsPanelTemplate(() => new StackPanel { Orientation = Orientation.Vertical });
+            new ItemsPanelTemplate(() => new StackPanel());
 
         public static readonly PerspexProperty<IEnumerable> ItemsProperty =
             PerspexProperty.Register<ItemsControl, IEnumerable>("Items");

Perspex.Controls/Primitives/TabStrip.cs

@@ -16,13 +16,9 @@ namespace Perspex.Controls.Primitives
         public static readonly PerspexProperty<TabItem> SelectedTabProperty =
             TabControl.SelectedTabProperty.AddOwner<TabStrip>();
 
-        private static readonly ItemsPanelTemplate PanelTemplate = new ItemsPanelTemplate(
-            () => new StackPanel());
-
         static TabStrip()
         {
             FocusableProperty.OverrideDefaultValue(typeof(TabStrip), false);
-            ItemsPanelProperty.OverrideDefaultValue(typeof(TabStrip), PanelTemplate);
         }
 
         public TabStrip()