Fix QSignalMapper connects silently broken under Qt6 (using the old string based system). QSignalMapper::mapped(int/QWidget*) was deprecated in Qt 5.15 and

removed in Qt6, replaced by mappedInt/mappedWidget/mappedString.

What was broken:
* the logo-conflict rename dialog
* the system tray show/hide toggle
* the Window menu
* export dialog's per-diagram preview controls

Switched to the modern mappedInt/mappedWidget signals with pointer-to-member connect(), guarded for Qt < 5.15 until Qt5 can be dropped.

sources/qetdiagrameditor.cpp

@@ -107,7 +107,11 @@ QETDiagramEditor::QETDiagramEditor(const QStringList &files, QWidget *parent) :
 	m_workspace.setTabsClosable(true);
 
 		//Set the signal mapper
+	#if QT_VERSION >= QT_VERSION_CHECK(5, 15, 0)
+	connect(&windowMapper, &QSignalMapper::mappedObject, this, [this](QObject *object) { activateWidget(qobject_cast<QWidget *>(object)); });
+	#else // TODO Qt6 only: remove, mappedObject() always available
 	connect(&windowMapper, SIGNAL(mapped(QWidget *)), this, SLOT(activateWidget(QWidget *)));
+	#endif
 
 	setWindowTitle(tr("QElectroTech", "window title"));
 	setWindowIcon(QET::Icons::QETLogo);