Mail Kit: Changed how filter changes work.

* BMailFilter::HeaderFetched() now only alters the entry_ref, and returns
  B_MOVE_MAIL_ACTION to move a mail.
* Instead of potentially moving the file around several times, the
  BMailProtocol now takes care of carrying out the filter action just once,
  including trying to make the file name unique.
* This also allows the IMAP add-on to know the final location of the mail,
  and thus downloading a message actually works.
* However, with my test inbox, it currently hangs, and a current Debugger does
  not work on my older system -- I guess I need to update.
* Replaced the duplicated space mechanism within the "HaikuMailFormatFilter"
  that is substantially faster, and handles all whitespace, not just spaces.
  It will also replace tabs with spaces.