(thumbs-buffer): New variable. Make it buffer local.

(thumbs-buffer): New variable.  Make it buffer local.
(thumbs-find-image): Move image name and number from buffer name
to mode name. Set thumbs-buffer.  Preserve point so that large
images remain visible.
(thumbs-file-alist): Construct list in thumbs-buffer and reverse
order.
(thumbs-show-image-num): Get image from thumbs-file-alist.  Set
mode name.
(thumbs-next-image, thumbs-previous-image): Make them work.