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/Tools/plugins/widget/customwidgets.cpp

@@ -143,7 +143,7 @@ FileChooser::FileChooser( QWidget *parent )
   : QWidget( parent ), md( File ), _filter( QString() )
 {
     QHBoxLayout *layout = new QHBoxLayout( this );
-    layout->setMargin( 0 );
+    layout->setContentsMargins( 0, 0, 0, 0 );
     layout->setSpacing( 6 );
 
     lineEdit = new QLineEdit( this );