Fix blurhash greedy decoding

QUrl::fromPercentEncoding destroys some valid base83 strings that contain encodings like %14, which Qt turns into Unicode garbage. Now, we use our own encoding table containing the only special ASCII characters that blurhash uses.

Yes, it's not pretty but it works. I couldn't find any easier way to write a dozen lines of QMap'd QLatin1String :-( I used QLatin1String because the Qt docs says its more efficient than using QStringLiteral, assuming the called function takes QLatin1String (and QString::replace does!)