Gui: Qt6 port

* QString::indexOf() is now marked as [[nodiscard]]
* Replace deprecated methods of QMessageBox
* QMouseEvent::globalPos() is deprecated, use globalPosition().toPoint()
* QWidget::enterEvent() requires a QEnterEvent as argument
* QLibraryInfo::location() is deprecated, use path()
* QVariant::Type is deprecated, use QMetaType::Type
* QVariant::canConvert(int) is deprecated, use QVariant::canConvert(QMetaType) or QVariant::canConvert<T>()
* QMessageBox::standardIcon is deprecated, use QStyle::standardIcon()
* Replace deprecated method QMessageBox::question(), ...
* QApplication::fontMetrics() is deprecated
* QDropEvent::mouseButtons() is deprecated, use buttons()
* QDropEvent::keyboardModifiers() is deprecated, use modifiers()
* Constructor of QFontDatabase is deprecated, use static methods instead
* Qt::AA_DisableHighDpiScaling is deprecated
* Qt::AA_EnableHighDpiScaling is deprecated
* Qt::AA_UseHighDpiPixmaps is deprecated

src/Gui/Application.cpp

@@ -1866,18 +1866,24 @@ void Application::runApplication()
 #ifdef FC_OS_WIN32
         SetProcessDPIAware(); // call before the main event loop
 #endif
+#if QT_VERSION < QT_VERSION_CHECK(6,0,0)
         QApplication::setAttribute(Qt::AA_DisableHighDpiScaling);
+#endif
     }
     else {
         // Enable automatic scaling based on pixel density of display (added in Qt 5.6)
+#if QT_VERSION < QT_VERSION_CHECK(6,0,0)
         QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
+#endif
 #if QT_VERSION >= QT_VERSION_CHECK(5,14,0)
         QGuiApplication::setHighDpiScaleFactorRoundingPolicy(Qt::HighDpiScaleFactorRoundingPolicy::PassThrough);
 #endif
     }
 
+#if QT_VERSION < QT_VERSION_CHECK(6,0,0)
     //Enable support for highres images (added in Qt 5.1, but off by default)
     QCoreApplication::setAttribute(Qt::AA_UseHighDpiPixmaps);
+#endif
 
     // Use software rendering for OpenGL
     ParameterGrp::handle hOpenGL = App::GetApplication().GetParameterGroupByPath("User parameter:BaseApp/Preferences/OpenGL");