Webform integration

@chrisolof Looks good so far. I've created an issue fork and added your patch to it, plus a tiny change to add core/drupal.autocomplete as a library dependency, so ui_autocomplete isn't seen as null or undefined.

What I'd like to do next is write a crosswalk between xAL property names and each of the fields that we're autocompleting into. I'll keep the existing property mappings so current implementations don't change.

I've got a few more feature ideas, but they might be better off kept to other requests.

I've done a pass at the merge request. One thing that occurs to me is that we might want the crosswalk to be configurable, depending on the autocomplete service that's being used. Do you think we should make that a setting instead, with some sensible defaults?