Multi-line CompositeListItem

At the moment, CompositeListItem derives from BoxLayout, which only allows a 1D line of children. I have a fairly large number of fields that I want to display per item, and to do that I have derived from GridLayout instead of BoxLayout and defaulted to rows=1. This will break user code that is currently depending on it deriving from BoxLayout. I'm not sure how severe that would be. Alternative suggestions welcome :)