Add a non template member QDjango::metaModel

[ericLemanissier]

Adds a non template member QDjangoMetaModel QDjango::metaModel(const QObject *model)

It avoids the use of template to retrieve an already registered model, and as a consequence allows one to develop a non template function that saves any QObject in database:

bool storageManager::saveItem(QObject *o)
{
    QDjangoMetaModel meta = QDjango::metaModel(o);
    if(!meta.isValid())
    {
        qWarning() << "Metamodel is not available in storageManager::saveItem for" << o->metaObject()->className();
        return false;
    }
    for(const auto &i : meta.foreignFields().keys())
    {
        meta.setForeignKey(o, i, o->property(i).value<QObject*>());
    }
    return meta.save(o);
}

This is especially useful when using QDjango without QDjangoModel