IPTC and Utf8 support: If a tag is string, check if global IPTC characterset...

IPTC and Utf8 support: If a tag is string, check if global IPTC characterset is null to convert in latin1, else we expect to interpret the string as utf8.
We use std::string accessor from Exiv2 to get an Utf8 cenversion of string. If it do not work, well this problem need to be reported as UPSTREAM
to Exiv2 as pre-cenversion of string is not done in background by the library.
This patch prevent to display latin1 string with a wrong Utf8 conversion which can break some characters.
BUGS: 379581
BUGS: 379050
FIXED-IN: 7.1.0