Make ShowmessageAction not be a subclass of MailboxAction

Make ShowmessageAction not be a subclass of MailboxAction

The ShowmessageAction was using the MailboxAction to do its display of
a single direct message. Since we redid the nickname management, this
was breaking (MailboxAction requires a nickname argument,
ShowmessageAction does not, and nickname validation that used to
quietly fail now throws an exception).

I've moved the message list processing to its own widget class, so the
need to subclass MailboxAction has disappeared. I've rewritten this
action to use the MessageListItem widget, and it works fine now.