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/ngldialogbuttonbox.h

@@ -1,6 +1,6 @@
 #pragma once
 
-#include <QtCore/QMap>
+#include <QtCore/QMultiMap>
 #include <QWidget>
 
 #include "nglwidgets_export.h"
@@ -177,7 +177,7 @@ protected Q_SLOTS:
   void buttonClicked();
 
 protected:
-  QMap<qint32, NGLStandardButton *> m_standardButtons;
+  QMultiMap<qint32, NGLStandardButton *> m_standardButtons;
   Qt::Orientation m_orientation;
   NGLStandardButton::StandardButtons m_buttons;
   NGLStandardButton::StandardButtons m_disabledActions;