Previously the headers-config was only applied to gmail-mailboxes (I
guess that was a mistake of my past). This change configures them for
all mailboxes.
Furthermore I dropped the x-mailer, since I'm really _not_ interested in
the sending mail-client...
Finally I adjusted the order of the headers, from the, sometimes,
awkward default-ordering to my own preferences.
Reference: http://www.fefe.de/muttfaq/muttrc
