gnus-art.el: Misc improvements for displaying MIME parts

gnus-art.el: Misc improvements for displaying MIME parts

* gnus-art.el (gnus-mm-display-part): Don't put article out of sight
while prompting a user for a file name, etc.
(gnus-mime-display-single): Display part with a common appearance no
matter whether MIME button is omitted or not; don't add duplicate entry
to gnus-article-mime-handle-alist.
(gnus-mime-buttonize-attachments-in-header): Use copied buttons.