Initial implementation of UI anchors, update TestGame.

Voile/Source/UI/IElement.cs

@@ -88,4 +88,11 @@ public interface IInputElement
     /// </summary>
     /// <param name="action">Input action to send.</param>
     void Input(UIInputContext action);
+}
+
+public interface IAnchorableElement
+{
+    public Anchor Anchor { get; set; }
+    public Vector2 AnchorOffset { get; set; }
+    public void ApplyAnchor(Vector2 parentPosition, Rect parentRect);
 }