(gnus-article-display-x-face-with-x-face-mule): Do nothing if `window-system'

(gnus-article-display-x-face-with-x-face-mule): Do nothing if `window-system'
is nil.
(gnus-article-decode-message-body-as-default-mime-charset): Don't bind
`buffer-read-only'; decode from (point-min) instead of (point).
(gnus-article-prepare-display): Bind `inhibit-read-only' to t; bind
`buffer-read-only' to nil; don't bind `mime-preview-over-to-next-method-alist';
strip read-only properties after preparing.
(gnus-article-prepare-mime-display): Put text property `article-treated-header'
to the header; don't use `mime-preview-move-to-next'; reduce a number of bound
variables.
(gnus-article-display-traditional-message): Don't bind `inhibit-read-only'.
(gnus-article-make-menu-bar): Use `gnus-article-toggle-headers' instead of
`gnus-article-hide-headers'.
(article-toggle-headers): New function.