qt6: nglcompression, widgets, dialog fixes: remove QTextCodec, QStringRef/splitRef, QDesktopWidget, use QMultiMap and Qt6 APIs; adjust QtConcurrent::run wrappers; replace endl with newline

Z/src/nglwidgets/nglpdfwidget.h

@@ -2,7 +2,8 @@
 
 #pragma once
 #include <QWidget>
-#include <poppler/qt5/poppler-qt5.h>
+#include <memory>
+#include <poppler/qt6/poppler-qt6.h>
 
 #include "nglwidgets_export.h"
 
@@ -32,7 +33,7 @@ public:
   int curPage() const;
 
 Q_SIGNALS:
-  void currentPageChanged(int cur, int max);  
+  void currentPageChanged(int cur, int max);
   void rotationChanged(Poppler::Page::Rotation newRotation);
 protected Q_SLOTS:
   void pageChanged(int cur, int max);
@@ -42,7 +43,7 @@ protected Q_SLOTS:
   void onZoomMinus();
   void onZoomPlus();
 protected:
-  double m_zoomFactor;
-  Ui::pdfWidget *ui;
+  double m_zoomFactor{1.0};
+  std::unique_ptr<Ui::pdfWidget> ui;
 };
-}       
+}