* WL-ELS (ELMO-MODULES): Added elmo-signal.

* elmo-signal.el: New file.

* elmo.el (toplevel): Require elmo-signal.
(flag-changing, flag-changed, cache-changed): New signal.
(elmo-folder): Remove slog `handlers'.
(elmo-message-set-cached): Use `elmo-emit-signal' instead of
`elmo-folder-notify-event'.
(elmo-folder-set-flag): Likewise. Emit signal `flag-changing'.
(elmo-folder-unset-flag): Ditto.
(elmo-event-handler): Abolish.
(elmo-event-handler-flag-changed): Ditto.
(elmo-event-handler-cache-changed): Ditto.
(elmo-folder-add-handler): Ditto.
(elmo-folder-remove-handler): Ditto.
(elmo-folder-notify-event): Ditto.

* elmo-pipe.el (toplevel): Require elmo-signal.
(elmo-folder-initialize): Call `elmo-pipe-connect-signals'.
(elmo-pipe-connect-signals): New function.
(elmo-message-fetch): Does not call `elmo-folder-notify-event'.
(elmo-folder-set-flag): Ditto.
(elmo-folder-unset-flag): Ditto.
(elmo-message-set-cached): Ditto.

* elmo-multi.el (toplevel): Require elmo-signal.
(elmo-folder-initialize): Call `elmo-multi-connect-signals'.
(elmo-multi-connect-signals): New function.
(elmo-multi-map-numbers): Ditto.
(elmo-message-set-cached): Does not call `elmo-folder-notify-event'.
(elmo-message-fetch): Ditto.
(elmo-folder-set-flag): Ditto.
(elmo-folder-unset-flag): Ditto.

* elmo-filter.el (toplevel): Require elmo-signal.
(elmo-folder-initialize): Call `elmo-filter-connect-signals'.
(elmo-filter-connect-signals): New function.
(elmo-filter-add-flag-count): Ditto.
(elmo-message-fetch): Does not call
`elmo-filter-folder-countup-message-flags' and
`elmo-folder-notify-event'.
(elmo-message-set-cached): Ditto.
(elmo-folder-set-flag): Ditto.
(elmo-folder-unset-flag): Ditto.

* wl.el (wl-exit): Call `elmo-clear-signal-slots'.

* wl-summary.el (wl-summary-buffer-event-handler): Abolish.
(wl-summary-event-handler): Ditto.
(wl-summary-update-persistent-mark-on-event): New
function (renamed from `elmo-event-handler-flag-changed').
(wl-summary-buffer-attach): New function.
(wl-summary-buffer-detach): Rewrite by `elmo-signal'.
(wl-summary-buffer-set-folder): Use `wl-summary-buffer-attach'
instead of `elmo-folder-add-handler'.