This should (probably) be public.  I frequently use this method to deserialize the XML/JSON that it stores into a `MultiUrlPickerState` object (same with `UrlPickerState`).  Makes working in Razor so much easier.

This could probably be abstracted away using a `PropertyEditorValueConverter`, but I still think it's nice to have for folks who wish to roll their own.

Not sure I can't think of a situation where anyone would need `Serialize`, though.  Lee or others may know better.