No support for listing of unseen messages per mailbox (not per server)

Plugin has messed basic terminology: mailbox is a folder on a server (due to rfc):

   The Internet Message Access Protocol Version 4rev2 (IMAP4rev2) allows
   a client to access and manipulate electronic mail messages on a
   server.  IMAP4rev2 permits manipulation of mailboxes (remote message
   folders)

So bulk renaming is necessary.

And xfce_mailwatch_signal_new_messages() pass total per server new_messages
count into _XfceMailwatch.mailboxes instead of _XfceMailwatch.servers.mailboxes.