Clarify use of qPrintable together with qDebug and friends (toLocal8Bit() vs utf8)

qPrintable() is defined as "str.toLocal8Bit().constData()". One of it's main uses (also documented) is actually to allow printing of QStrings in qDebug statements, e..g

qWarning("%s: %s", qPrintable(key), qPrintable(value));

However, qWarning in the end uses QString::vsnprintf() to format it's output, which - according to the documentation of QString::sprintf() - expects "UTF-8 string for %s and Latin-1 string for the format string". The QDebug::operator<< for const char * also uses QString::fromUtf8(t).

I suppose we can

• fix QDebug, qt_message to expect local 8 bit encoding for const char *.
• deprecate the use of qPrintable in combination with qDebug.
• redefine qPrintable to str.toUtf8().constData(), and break it for people using it for system printf()/cout.