Port RFC 2047 decoder away from QTextCodec
This is mostly using QStringDecoder as a direct replacement, apart from the final fallback logic to the local codec. That has been removed as the local codec is assumed to be UTF-8 in Qt 6 anyway.
This is mostly using QStringDecoder as a direct replacement, apart from the final fallback logic to the local codec. That has been removed as the local codec is assumed to be UTF-8 in Qt 6 anyway.