Major port Qt4 to Qt6

- Updated header inclusions from QtGui to QtWidgets for consistency and compatibility with Qt 5 and later.
- Replaced deprecated QApplication::desktop() with QApplication::primaryScreen() for obtaining screen DPI.
- Modified MyOutputHandler to use QMessageLogContext and QString for improved logging.
- Changed QMatrix to QTransform for image rotation in ImageCutout class.
- Updated various debug statements to use toLocal8Bit() instead of toAscii() for better string handling.
- Refactored several classes to use QElapsedTimer instead of QTime for performance measurement.
- Ensured all operator overloads in Section and Tag classes are marked as const for better const-correctness.

ClippingStation/pagemargin.cpp

@@ -20,8 +20,8 @@
 #include "pagemargin.h"
 
 #include <QPen>
-#include <QDesktopWidget>
 #include <QApplication>
+#include <QScreen>
 #include <QGraphicsLineItem>
 
 #include "constants.h"
@@ -31,7 +31,7 @@ PageMargin::PageMargin(bool isfirst,QGraphicsRectItem * parent)
        :     QGraphicsRectItem(parent)
 {
     // we compute pixels dimensions for the page.
-    int dpi = QApplication::desktop()->logicalDpiX();
+    int dpi = QApplication::primaryScreen()->logicalDotsPerInchX();
     qreal pageWidth = dpi * A4_WIDTH;
     qreal pageHeight = dpi * A4_HEIGHT;