Fix scroll presenter child sizing.

Fixes #34.

Perspex.Controls/Presenters/ScrollContentPresenter.cs

@@ -95,7 +95,7 @@ namespace Perspex.Controls.Presenters
 
             if (child != null)
             {
-                child.Arrange(new Rect((Point)(-this.Offset), finalSize));
+                child.Arrange(new Rect((Point)(-this.Offset), child.DesiredSize.Value));
                 return finalSize;
             }
 

Perspex.Themes.Default/ListBoxStyle.cs

@@ -9,6 +9,7 @@ namespace Perspex.Themes.Default
     using System.Linq;
     using Perspex.Controls;
     using Perspex.Controls.Presenters;
+    using Perspex.Layout;
     using Perspex.Media;
     using Perspex.Styling;