Removing new trade hotkeys while a counteroffer is displayed.

Fixes http://www.bay12forums.com/smf/index.php?topic=126076.1140
It's tempting to use has_offer for this effect, but that flag remains true after hitting Enter to consider the offer.
Relies on a structure update: https://github.com/DFHack/df-structures/pull/18
Moving trade screen search dimming to the proper row.

Unfortunately, this now means it hits one of the search clear lines when both are active.
Merge pull request #316 from eswald/trade-search

Removing new trade hotkeys while a counteroffer is displayed.