Create and use MessageList widget

Create and use MessageList widget

Our mailbox actions (inbox and outbox) were doing their own display of
messages. This was causing issues with especially showmessage, which
since the more rigourous nickname checks were added, no longer works as
a mailbox subclass.

I've taken the time to rip out the message listing code from MailboxAction
and moved it to a MessageList widget. The different mailboxes now have their
own subclasses that show the correct profile in the list.