Qt6 support for eDA

[rozyczko]

Qt6 is now pretty well established, we need to spend time to upgrade the App (easyApp and easyDiffractionApp and easyReflectometryApp) to the newest version of QML and PySide2

[AndrewSazonov]

There are a few things that are missing in Qt6. The main one is related to the `XmlListModel' we use as our data model.

• In Qt5 XmlListModel had a property called xml which stored the XML data for this model as a string (see https://doc.qt.io/qt-5/qml-qtquick-xmllistmodel-xmllistmodel.html).
• In Qt6 this property is missing (see https://doc.qt.io/qt-6/qml-qtqml-xmllistmodel-xmllistmodel.html).

Further research is needed to find out whether Qt developers plan to add this missing functionality or if we need to implement our own `XmlListModel'.