also:
* use implicit conversion from toLatin1-QByteArray to const char*, where possible
* constData() instead of data() everywhere, to avoid unneeded deep copies if code get copied
* removed unneeded conversion to latin1 where QString was expected
* use QByteArray instead of QString, if QString not really needed
* cache result of conversion where useful

REVIEW: 107871

Thanks boud and jarosław for review