Edited Feature: PropertyGrid : Expanding Collections to show children [22228]

Based on codeplex discussion https://wpftoolkit.codeplex.com/discussions/653034.

User wants to be able to use a Collection <T> as a PropertyItem in the PropertyGrid and when expanding this PropertyItem, see the Collection children, the T items.
Currently, expanding a Collection<T> will show the Collection properties, which is its Count.