Bug 800402 - [email] UI: Show the number of unread messages in the message-list header after the folder name

At one point you talked about not using this function; if you still do, kill the `dump` on line 685 and maybe add a brief docstring, something like "Expect the number of unread messages in the folder to to equal num." And I might rename `name` to `desc`, only because `name` seems to imply that the name is used for something important and needs to conform to some convention, when in reality it's just a way to tag the expectation for logging purposes.