(gnus-article-display-method-for-mime): New variable; abolish

(gnus-article-display-method-for-mime): New variable; abolish
`gnus-show-mime-method'.
(gnus-article-display-method-for-encoded-word): New variable; abolish
`gnus-decode-encoded-word-method'.
(gnus-article-display-method-for-traditional): New variable.
(gnus-article-display-mime-message): New function; abolish
`gnus-article-preview-mime-message'.
(gnus-article-display-traditional-message): New function.
(gnus-article-display-message-with-encoded-word): New function;
abolish `gnus-article-decode-encoded-word'.
(gnus-article-prepare): Change display mechanism; use
`gnus-request-original-article' instead of
`gnus-request-article-this-buffer'.
(gnus-request-original-article): New function.