* List the available IMAP folders in the mailbox properties dialog

* Allow to set a different mail reader for each mailbox. Invoking the
  mail reader action would run the mail reader of the most recent
  message's mailbox, or fallback to the global mail reader.

* Limit the tooltip height to the screen height

* The "delete" message action could also be implemented with little
  effort for Maildir and Sylpheed mailboxes (other mailbox types would
  require more work).
