(wl-thread-entity-get-linked): New function.

(wl-thread-entity-get-linked): New function.
(wl-thread-entity-set-linked): New function.
(wl-thread-create-entity): Add linked element.
(wl-thread-entity-insert-as-top): Use wl-append.
(wl-thread-maybe-get-children-num): If closing thread, return
children msgs.
(wl-thread-update-line-msgs): Displaying progress message.
(wl-thread-update-line-on-buffer-sub): Use
wl-thread-maybe-get-children-num.
(wl-thread-update-line-on-buffer): If update line is not exists,
insert thread.
(wl-thread-delete-message): If delete top msg of thread, search
parent by subject.
(wl-thread-insert-entity): Use wl-thread-maybe-get-children-num.