Set destination using search text if exact match is found

If a player hits `` while in a destination search field _and_ their search text is an exact match for a zone/spaceship name _and_ that zone/spaceship exists in the dropdown (ie. is a valid target for the relevant entity/force), then set the destination to the player's search text.

Modified from original suggestion by stringweasel:
> When searching a destination, if there is only one match left, automatically select it. Keep the cursor in the search though. It's quite annoying have to type in name, having only the one location left, but still having to click on the drop down to select it.