(www-image-write-html): Add new argument `dest-dir'; change order of

arguments; expect `prev-file', `file' and `next-file' are relative
path; expect `prev-file' and `next-file' are omitted extensions.
(www-image-convert-images): Add new argument `dest-dir'; change order of
arguments; change `prev-file' and `next-file' to non-optional
arguments; modify for `www-image-write-html'.