Allow applying filters to summary consecutively

Allow applying filters to summary consecutively

* lisp/mail/rmailsum.el (rmail-summary-currently-displayed-msgs):
New variable.
(rmail-summary-fill-displayed-messages, rmail-summary-negate): New
functions.
(rmail-summary-by-labels, rmail-summary-by-recipients)
(rmail-summary-by-regexp, rmail-summary-by-topic)
(rmail-summary-by-senders): Accept an additional argument
KEEP-FILTERING, to narrow the existing filtered summary.

* etc/NEWS: Announce the change.