issue #22980: performance improvements for K+H

I don't think we have a good widget for ItemSourceManufacturer entry. I don't want to cache this list (which rules out pickers and comboboxes), but I want to allow free-form entry (which rules out RelationWidgets). So I used a simple XV.InputWidget, which loses the whole point of having a collection.

It would be useful here to have a combobox that does not need to be backed by a *cached* collection.