Draft variant for save function. It's in gists because there is no task for it.

gistfile1.txt

namespace {
    template <typename T>
    bool appendToJsonInTextRepresentation(const T& valToAppend, const char * whereToAppend, std::string& str)
    {
        int positionOfFilesKey = str.find(whereToAppend);

        int positionOfClosingBracket = str.find (string("]"), positionOfFilesKey);

        switch (str.at(positionOfClosingBracket - 1))
        {
            case ' ':
            {
                str.insert(positionOfClosingBracket - 1, (string(", \"") + valToAppend).append("\""));
            }
            break;
            case '\"':
            {
                str.insert(positionOfClosingBracket, (string(", \"") + valToAppend).append("\""));
            }
            break;
            default:
            {
                return false;
            }
            break;
        }
        return true;
    }
}

void LibprojPlugin::saveProjectData(const string & WhatToAppend, const string & WhereToAppend)
{
    qDebug() << QString(WhatToAppend.c_str());
    string
            dumpedData = project->getProjectData().dump(),
            errors;
    bool result = appendToJsonInTextRepresentation<string>(WhatToAppend, WhereToAppend.c_str(), dumpedData);
    if (result) {
       project->setProjectData(Json::parse(dumpedData, errors));
    }
    else
        qWarning() << "Error when modifying json representation";
    qDebug() << QString(errors.c_str());

    QFile projectFile(projectFilename);
    if(!projectFile.open( QIODevice::ReadWrite | QIODevice::Text ))
        qDebug() << "File unsuccessfully opened";
    else
        qDebug() << "File successfully opened";
    QTextStream outStream(&projectFile);
    outStream << dumpedData.c_str();
    projectFile.close();
}