(gnus-show-mime-method): Use `gnus-article-preview-mime-message'

(gnus-show-mime-method): Use `gnus-article-preview-mime-message'
instead of `metamail-buffer' in default.
(gnus-decode-encoded-word-method): Use
`gnus-article-decode-encoded-word' instead of
`gnus-article-de-quoted-unreadable' in default.

Abolish `gnus-hack-decode-rfc1522', `gnus-decode-rfc1522',
`article-decode-rfc1522', `article-de-quoted-unreadable',
`article-mime-decode-quoted-printable-buffer' and
`article-mime-decode-quoted-printable'.
(gnus-article-decode-rfc1522): New implementation (use
`eword-decode-header').

(gnus-article-preview-mime-message): New function.
(gnus-article-decode-encoded-word): New function.
(gnus-content-header-filter): New function.
(mime-view-quitting-method-for-gnus): New function.
Add setting for mime-view.