Fixup 3dc54d4a: fix string encoding issues with Qt4

Fixup 3dc54d4a: fix string encoding issues with Qt4

The culprit here is the constructor QString(QByteArray const &): in
Qt4, it would interpret the byte array as latin1, and in Qt5 as utf8.

Therefore it is safer to use explicitly QString::fromUtf8 instead of
this constructor.

Several places where additionally simplified, in order to avoid some
extra conversions.