(mime-preview-multipart/mixed): Modify for `mime-view-display-entity';

(mime-preview-multipart/mixed): Modify for `mime-view-display-entity';
don't refer `mime-raw-buffer'.
(mime-preview-multipart/alternative): Modify for
`mime-view-display-entity'; don't refer `mime-raw-buffer'.
(mime-view-display-entity): Change interface to abolish argument for
raw-buffer; don't refer `mime-raw-buffer'.
(mime-view-display-message): Abolish variable
`mime-preview-original-major-mode'; modify for
`mime-view-display-entity'.
(mime-preview-original-major-mode): New implementation; add optional
argument `recursive'.
(mime-preview-follow-current-entity): Modify for
`mime-preview-original-major-mode'.
(mime-preview-move-to-next): Use function
`mime-preview-original-major-mode'.
(mime-preview-scroll-up-entity): Use function
`mime-preview-original-major-mode'.
(mime-preview-scroll-down-entity): Use function
`mime-preview-original-major-mode'.
(mime-preview-quit): Use function `mime-preview-original-major-mode'.
(mime-preview-show-summary): Use function
`mime-preview-original-major-mode'.