Add typeahead behavior to combo field

[bhousel]

The raw tag editor key and value fields combos will fire multiple debounced taginfo queries as the user types. This is a nice way to narrow the user's search and show better autocompletion values.

I was surprised to find that the combo preset field doesn't do this - it just fetches 25 values one time. So I'm going to change combo.js to work more like raw_tag_editor.js.

BTW, this matters a lot because cuisine=barbecue is not in the top 25, and people shouldn't need to use the raw tag editor to add it. 🍖

(This enhancement will get rolled into #3080 but it is not directly related to the multiselection ui field)
re: https://github.com/openstreetmap/iD/issues#issuecomment-214876017