respect non-ascii encodings in Launch target file names

PDF2.0 (and also 1.7) - section 7.11.3 specifies the UF field as a "text string" in the pdf sense, which might be encoded in one of three encodings. The encoding was ignored here before resulting in broken links when (1) the UF field was actually used and (2) the contents was a marked UTF16 string that would be interpreted as UTF8.

This particular issue gave me headaches for a few years now.

I considered to adjust the parameters too, but these are not officially defined in the standard as text -either "undefined" or partly as "byte string"- and have been deprecated in PDF2.0, so I think it's better not to break anything that's not reported to be broken.