Make SelectingItemsControl abstract.

As it displays nothing by default.

Perspex.Controls/Primitives/SelectingItemsControl.cs

@@ -12,7 +12,7 @@ namespace Perspex.Controls.Primitives
     using Perspex.Input;
     using Perspex.VisualTree;
 
-    public class SelectingItemsControl : ItemsControl
+    public abstract class SelectingItemsControl : ItemsControl
     {
         public static readonly PerspexProperty<object> SelectedItemProperty =
             PerspexProperty.Register<SelectingItemsControl, object>("SelectedItem");

Tests/Perspex.Controls.UnitTests/SelectingItemsControlTests.cs

@@ -8,7 +8,6 @@ namespace Perspex.Controls.UnitTests
 {
     using Perspex.Controls.Primitives;
     using Perspex.Input;
-    using Perspex.Interactivity;
     using Xunit;
 
     public class SelectingItemsControlTests
@@ -16,7 +15,7 @@ namespace Perspex.Controls.UnitTests
         [Fact]
         public void PointerPressed_Event_Should_Be_Handled()
         {
-            var target = new SelectingItemsControl();
+            var target = new Target();
 
             var e = new PointerPressEventArgs
             {
@@ -27,5 +26,9 @@ namespace Perspex.Controls.UnitTests
 
             Assert.True(e.Handled);
         }
+
+        private class Target : SelectingItemsControl
+        {
+        }
     }
 }