API for excluding types

[JohanLarsson]

For example CaretElement makes little sense to track. Hacking it like this for now.

if (d.GetType().FullName == "System.Windows.Documents.CaretElement")
{
    SetForInputTypes((FrameworkElement)d, null);
    return;
}

Should probably be configurable via attached property with a sensible default.
Not sure how to specify inheritance, maybe:

IgnoredTypes=">Shape, System.Windows.Documents.CaretElement"
where >Shape would mean ignore subclasses of shape.