Add a public IList<IView> Children to new Layout base class

[hartez]

Modifies the Layout class for the new layouts to expose a public IList<IView> Children { get; } for compatibility with XAML and the ContentProperty attribute.

The implementation of the IList interface runs all of the add/remove/insert/clear methods through the same bookkeeping previously used, so Parenting and layout invalidation are still happening.

We'll get an alternate discussion started on whether ILayout/IContainer still need to provide IReadOnlyList<IView> Children { get; }, or whether we can simply move to IContainer : IList<IView>.