QTextCodec subclasses must reimplement this function.
Converts the first len characters of chars from the encoding of the subclass to Unicode, and returns the result in a QString.
state can be 0, in which case the conversion is stateless and default conversion rules should be used. If state is not 0, the codec should save the state after the conversion in state, and adjust the remainingChars and invalidChars members of the struct.
Referenced by toUnicode().