[API] Deprecate QString::fromAscii, QString::toAscii, add QString::fromCString, QString::toCString

Please consider deprecating QString::fromAscii, QString::toAscii in Qt5, and replace them by properly named QString::fromCString, QString::toCString methods.

As the documentation already states, fromAscii, toAscii isn't actually expecting or generating ASCII character strings. Instead, it's using whatever encoding is specified by QTextCodec::setCodecForCStrings() (which happens to have Latin-1 as default). If you set an ASCII incompatible codec it can happen that fromAscii, toAscii actually rejects QByteArray's in ASCII.