#13 add ability to recall a queued message

[Joe Strout]

Zymail queues messages for 3 minutes before sending them very nicely. But you can't edit a message that's queued; you have to either delete it entirely, or let it go out as-is.

Add the ability to reopen a queued message for further editing, either by double-clicking it or selecting it and pressing Enter/Return. The message should be dequeued (removed from the Outbox), and opened in a Compose window exactly as it was before. The user can then edit and Send again, or close the window without saving (which deletes it).