Qt6 port:

* QApplication::setFallbackSessionManagementEnabled has been removed
* QString::medRef() has been removed. Use QString::mid() again.
* QTextStream::setCodec has been removed
* Use operator QVariant of the QFont class to make code Qt5 and Qt6 compatible
* Signature of QTreeWidget::mimeData() has changed in Qt6. Remove TreeWidget::mimeData() because it doesn't change the implementation
* QLayout::setMargin() is deprecated in Qt5 and has been removed in Qt6. Use QLayout::setContentsMargins()
* QDateTime::toTime_t() is deprecated in Qt5 and has been removed in Qt6. Use QDateTime::toSecsSinceEpoch()
* QDesktopWidget is deprecated in Qt5 and has been removed in Qt6. Use QScreen

src/Gui/GuiApplication.cpp

@@ -62,7 +62,9 @@ GUIApplication::GUIApplication(int & argc, char ** argv)
 {
     connect(this, SIGNAL(commitDataRequest(QSessionManager &)),
             SLOT(commitData(QSessionManager &)), Qt::DirectConnection);
+#if QT_VERSION < QT_VERSION_CHECK(6,0,0)
     setFallbackSessionManagementEnabled(false);
+#endif
 }
 
 GUIApplication::~GUIApplication()