Improve directional naviation a bit.

In SelectingItemsControl. Still not completely there as we should do a
visual search in the absence of INavigablePanel (e.g. for Canvas, Grid).
Fixes half of #44.

Perspex.Controls/INavigablePanel.cs

@@ -0,0 +1,15 @@
+// -----------------------------------------------------------------------
+// <copyright file="INavigablePanel.cs" company="Steven Kirk">
+// Copyright 2014 MIT Licence. See licence.md for more information.
+// </copyright>
+// -----------------------------------------------------------------------
+
+namespace Perspex.Controls
+{
+    using Perspex.Input;
+
+    public interface INavigablePanel
+    {
+        Control GetControl(FocusNavigationDirection direction, Control from);
+    }
+}

Perspex.Controls/Perspex.Controls.csproj

@@ -40,6 +40,7 @@
     <Compile Include="DropDown.cs" />
     <Compile Include="IContentControl.cs" />
     <Compile Include="IItemsPanel.cs" />
+    <Compile Include="INavigablePanel.cs" />
     <Compile Include="Platform\IPopupImpl.cs" />
     <Compile Include="Platform\ITopLevelImpl.cs" />
     <Compile Include="Popup.cs" />

Perspex.Controls/Primitives/SelectingItemsControl.cs

@@ -65,33 +65,25 @@ namespace Perspex.Controls.Primitives
 
         protected virtual void MoveSelection(FocusNavigationDirection direction)
         {
-            if (this.SelectedItem != null)
+            var panel = this.Presenter?.Panel as INavigablePanel;
+            var selected = this.SelectedItem;
+            var container = selected != null ?
+                this.ItemContainerGenerator.GetContainerForItem(selected) :
+                null;
+
+            if (panel != null)
             {
-                int offset = 0;
+                var next = panel.GetControl(direction, container);
 
-                switch (direction)
+                if (next != null)
                 {
-                    case FocusNavigationDirection.Up:
-                    case FocusNavigationDirection.Left:
-                        offset = -1;
-                        break;
-                    case FocusNavigationDirection.Down:
-                    case FocusNavigationDirection.Right:
-                        offset = 1;
-                        break;
-                }
-
-                if (offset != 0)
-                {
-                    var currentIndex = GetIndexOfItem(this.SelectedItem);
-                    var index = currentIndex + offset;
-
-                    if (index >= 0 && index < this.Items.Cast<object>().Count())
-                    {
-                        this.SelectedItem = this.Items.Cast<object>().ElementAt(index);
-                    }
+                    this.SelectedItem = this.ItemContainerGenerator.GetItemForContainer(next);
                 }
             }
+            else
+            {
+                // TODO: Try doing a visual search?
+            }
         }
 
         protected override void OnKeyDown(KeyEventArgs e)

Perspex.Controls/StackPanel.cs

@@ -7,10 +7,8 @@
 namespace Perspex.Controls
 {
     using System;
-    using System.Collections.Generic;
     using System.Linq;
-    using System.Text;
-    using System.Threading.Tasks;
+    using Perspex.Input;
 
     public enum Orientation
     {
@@ -18,7 +16,7 @@ namespace Perspex.Controls
         Horizontal,
     }
 
-    public class StackPanel : Panel
+    public class StackPanel : Panel, INavigablePanel
     {
         public static readonly PerspexProperty<double> GapProperty =
             PerspexProperty.Register<StackPanel, double>("Gap");
@@ -38,6 +36,49 @@ namespace Perspex.Controls
             set { this.SetValue(OrientationProperty, value); }
         }
 
+        Control INavigablePanel.GetControl(FocusNavigationDirection direction, Control from)
+        {
+            var horiz = this.Orientation == Orientation.Horizontal;
+            int index = this.Children.IndexOf(from);
+
+            switch (direction)
+            {
+                case FocusNavigationDirection.First:
+                    index = 0;
+                    break;
+                case FocusNavigationDirection.Last:
+                    index = this.Children.Count - 1;
+                    break;
+                case FocusNavigationDirection.Next:
+                    ++index;
+                    break;
+                case FocusNavigationDirection.Previous:
+                    ++index;
+                    break;
+                case FocusNavigationDirection.Left:
+                    index = horiz ? index - 1 : -1;
+                    break;
+                case FocusNavigationDirection.Right:
+                    index = horiz ? index + 1 : -1;
+                    break;
+                case FocusNavigationDirection.Up:
+                    index = horiz ? -1 : index - 1;
+                    break;
+                case FocusNavigationDirection.Down:
+                    index = horiz ? -1 : index + 1;
+                    break;
+            }
+
+            if (index >= 0 && index < this.Children.Count)
+            {
+                return this.Children[index];
+            }
+            else
+            {
+                return null;
+            }
+        }
+
         protected override Size MeasureOverride(Size availableSize)
         {
             double childAvailableWidth = double.PositiveInfinity;