Combos don't always get placeholders from taginfo

[bhousel]

When you hover over an item, combo fields are created, and taginfo calls fire off to fetch the placeholders for those fields. But it seems like if you select the item by clicking on it before the callback is called, the field never gets a placeholder.

[bhousel]

Oh, it's probably because the requests are debounced
https://github.com/openstreetmap/iD/blob/master/modules/services/taginfo.js#L139