Migration to QT6
I've already committed several things that should deal with most of the qt6 compatibility. However, since qt5.13 the QtXmlPatterns class has been deprecated and it has been removed in Qt6. In the code, QAbstractMessageHandler is used to derive a class and QXmlSchema and QXmlSchemaValidator are used in the XMLReader class, in XMLReader::validateSchema().
If I understand correctly, all of this is "only" needed in order to offer schema validation, which we currently don't really do as our xsd is pretty much empty. So, it should be OK to just drop XMLReader::validateSchema() so we have no dependency to QtXmlPatterns anymore, at a latter stage populate the xsd and rely on external tools for schema validation?