Unify Containers and Widgets by creating a base UIElement, add more anchor types, make anchor calculations an extension of Anchor.

Voile/Source/UI/IElement.cs

@@ -39,7 +39,7 @@ public interface IResizeableElement
     /// <summary>
     /// Get a minimum rectangle size for this element.
     /// </summary>
-    public abstract Rect MinimumRect { get; }
+    public abstract Rect MinimumSize { get; }
 }
 
 public interface IUpdatableElement