Add required something

Add required something

fixed nui-376:
1. Change public window constructor to internal;
2. Chenge Vector2/4 to high level class Size2D and Color;
   public Vector2 Size
   public Vector4 BackgroundColor
3. Add some layer apis;
4. Add SynchronousLoading property for ImageView.

Change-Id: I1deff95eae359cafacfd399d0208ee4a713d582a