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/Splashscreen.cpp

@@ -412,7 +412,9 @@ void AboutDialog::showCredits()
     creditsHTML += QString::fromLatin1("</h2><ul>");
 
     QTextStream stream(&creditsFile);
+#if QT_VERSION < QT_VERSION_CHECK(6,0,0)
     stream.setCodec("UTF-8");
+#endif
     QString line;
     while (stream.readLineInto(&line)) {
         if (!line.isEmpty()) {