Torbutton should have a downward arrow menu hint

We've had issues with people mistaking Vidalia's New Identity with Torbutton's New Identity (https://lists.torproject.org/pipermail/tor-qa/2012-June/000014.html), which we hope that #5277 (closed) will improve.

However, the root issue might be a lack of any hint that Torbutton actually provides a menu now if you click on it.. A downward arrow next to the button should help provide that hint.