Basically a copy of qt5 to qt6

Tested with the tests and the demo, seems to work relatively well

Changes:
 * Changed a few QLinkedList to QVector, we don't need the features that
   QLinkedList provided
 * Adapt code to QByteArray behaviour change in [] with indexes past the
   size
 * Removed a few deprecated functions from our API
 * Use more modern cmake syntax to link against the libraries
 * QDate::toString is gone, use QLocale::toString with a date
 * Use the QDateTime variants of secsSinceEpoch instead of time_t