Use Exiv2::CommentValue to format Exif.Photo.UserComment metadatum

Converting the UserComment exif metadatum to string will result in its
direct/quasi-internal string representation of libexiv2, which may
include a "charset=..." prefix with the charset of the value.

Since we want the actual content/value of UserComment, and the
Exiv2::Value held by Exiv2::Exifdatum is Exiv2::CommentValue, then
cast it to call comment(). The result is converted to QString using
QString::fromStdString(), which converts std::string as UTF-8 string.

For further details, see also the exiv2 ticket:
Exiv2/exiv2#1258

Signed-off-by: Pino Toscano <toscano.pino@tiscali.it>

ImageLounge/src/DkCore/DkMetaData.cpp

@@ -469,7 +469,9 @@ QString DkMetaDataT::getNativeExifValue(const QString &key, bool humanReadable)
 
                 // qDebug() << "pos count: " << pos->count();
                 // Exiv2::Value::AutoPtr v = pos->getValue();
-                if (humanReadable) {
+                if (key == QLatin1String("Exif.Photo.UserComment")) {
+                    info = QString::fromStdString(static_cast<const Exiv2::CommentValue &>(pos->value()).comment());
+                } else if (humanReadable) {
                     std::stringstream ss;
                     ss << *pos;
                     info = exiv2ToQString(ss.str());