ControlledListItemViewer , ResourceInstanceViewer etc should only accept a single item, not an array

[chrabyrd]

No description provided.

[aarongundel]

I had started this, but realized there's an open question that we need to figure out about how to deal with multivalued items. Right now, these widgets are structured in such a way as to return a viewer or editor based on the mode provided to the widget. What do we do when a multivalued node needs a value set? We need to be able to set an array. Maybe there's some restructuring that can occur to make this happen, or we need to revisit how we're splitting the viewer and the editor.