Fix mangling names in temp directory (amends eef0c8e8).

Fix mangling names in temp directory (amends eef0c8e8).

Separate mangled-name tables for xhtml and latex exports.

LaTeX route should stay as it were (counter+mangled path within filenames in
temp directory)

xHTML route uses hash of path + filename only. We won't use counters because
(unlike in LaTeX export) they stay visible in final export directory and
generally change across LyX sessions.
Hash is short with collision handling because path-length limits of Windows.

cf https://www.mail-archive.com/lyx-devel@lists.lyx.org/msg220582.html