Gnus: Make gnus-mime-inline-part and gnus-mime-inline-part work similarly

Gnus: Make gnus-mime-inline-part and gnus-mime-inline-part work similarly

* gnus-art.el (gnus-mime-inline-part): Redisplay a button so as to show
the displaying state of a part.
(gnus-mime-inline-part): Don't insert a newline in the beginning of
a part like gnus-mime-inline-part doesn't; work for XEmacs.

* mm-decode.el (mm-display-part): Don't insert a newline in the top.
(mm-shr): Make undisplayer unbreakable.

* mm-view.el (mm-inline-image-emacs, mm-inline-image-xemacs): Don't
insert excessive newline.
(mm-inline-text-html-render-with-w3m, mm-inline-text)
(mm-insert-inline): Make undisplayer unbreakable.