(mime-preview-over-to-next-method-alist): Use `gnus-article-next-page' when

(mime-preview-over-to-next-method-alist): Use `gnus-article-next-page' when
the last page is not displayed.
(mime-preview-over-to-previous-method-alist): Use `gnus-article-prev-page'
when the first page is not displayed.
(gnus-next-page-map): Use `make-sparse-keymap' instead of `make-keymap';
don't use `suppress-keymap'.
(gnus-insert-next-page-button, gnus-insert-prev-page-button): Succeed to the
value of the text property `mime-view-situation' in the Next/Prev buttons;
make `gnus-{next|prev}-page-map' have the current local map under FSF Emacsen.