Get rid of QChar::toLower/toUpper calls in QFont* classes

Converting one QChar to lower/upper case is wrong for two reasons - QChar doesn't necessarily contain the whole character (think of surrogate pair), secondly QChar::toLower/toUpper function is not locale aware, and there is at least one locale (Turkish) where the uppercase of a latin character "i" is not latin "I", hence toUpper/toLower conversion should be locale-aware in that case.