Finished implemented Data classes.  In order for bindings to work properly, we still need Mentor, MentorChanged, and VisualParentChanged.
Added concept Mentor.  Mentor is a closest parent relative that is a FrameworkElement.  Mentor of a XamlObject can be itself.  Also added LogicalChildren.  Rather than walking the provider store to propagate mentor, we walk the logical children (these will be added when AttachTo is called).  Added ancestor changed listeners for BindingExpressionBase to consume.
ContentControl now falls back to ContentPresenter template.