* wl-summary.el (wl-summary-detect-mark-position): Use generic msgdb

* wl-summary.el (wl-summary-detect-mark-position): Use generic msgdb
for dummy entity.
(wl-summary-insert-thread): Use elmo-message-entity-field.
(wl-summary-update-thread): Use elmo-message-entity-field and
elmo-message-entity-number.
(wl-summary-line-number):  Use elmo-message-entity-number.
(wl-summary-line-size): Use elmo-message-entity-field.
(wl-summary-line-subject): Ditto.
(wl-summary-line-from): Ditto.
(wl-summary-line-attached): Ditto.
(wl-summary-create-line): Ditto.
(wl-summary-create-line): Use elmo-message-entity-number.
(wl-summary-print-message-with-ps-print): Use
elmo-message-entity-field.

* modb.el (elmo-msgdb-make-message-entity): Define simple one.
(elmo-msgdb-message-entity-field): Ditto.
(elmo-msgdb-message-entity-number): Ditto.