bug#29659: 25.2; better rmail-summary-by-topic

[Francesco Potortì]

This is my current attempt, seems to work, please give it a try.

(defun rmail-simplified-subject (&optional msgnum)
  "Return the simplified subject of message MSGNUM (or current message).
Simplifying the subject means stripping leading and trailing
whitespace, replacing whitespace runs with a single space and
removing prefixes such as Re:, Fwd: and so on and mailing list
tags such as [tag]."
  (let ((subject (or (rmail-get-header "Subject" msgnum) ""))
        (regexp "\`[ \t\n]*\\(\\(\\w\\{1,3\\}:\\|\\[[^]]+]\\)[ \t\n]+\\)*"))
    (setq subject (rfc2047-decode-string subject))
    (setq subject (replace-regexp-in-string regexp "" subject))
    (replace-regexp-in-string "[ \t\n]+" " " subject)))