* elmo-shimbun.el (elmo-shimbun-update-overview-folder-list): New

* elmo-shimbun.el (elmo-shimbun-update-overview-folder-list): New
user option.
(elmo-shimbun-header-extra-field): New inline function.
(elmo-shimbun-header-set-extra-field): Ditto.
(elmo-shimbun-folder-header-hash-setup): Ditto.
(elmo-shimbun-update-overview): Ditto.
(elmo-shimbun-msgdb-to-headers): Set extra field `x-shimbun-id'.
(elmo-shimbun-folder-setup): Use
`elmo-shimbun-folder-header-hash-setup' to be setup hash table.
(elmo-shimbun-get-headers): Ditto.
(elmo-map-message-fetch): Call `elmo-shimbun-update-overview';
Insert `X-Shimbun-Id:' header if extra field is set.
(elmo-map-folder-list-message-locations): Return `x-shimbun-id'
field's value instead of `shimbun-header-id'.

* elmo-msgdb.el (elmo-msgdb-overview-entity-set-references): New
inline function.
(elmo-msgdb-overview-entity-set-date): Ditto.
(elmo-msgdb-overview-entity-set-extra-field): Ditto.