workaround #2672 by avoiding the mechanism triggering the leak in PySide6 6.5.0

1. WidgetGroup was creating lambda closures with references to both self and widget.
   • This is a bad practice and should be avoided.
   • However, prior to PySide6 6.5.0, this didn't cause a leak. (although it would not have been surprising if it had!)
2. PlotItem leak since PySide6 6.5.0 #2672 (comment) demonstrates that the leak occurs even with a lambda that has no closures.
   • i.e. various techniques using weakrefs would still have leaked
   • further testing shows that using custom def functions (with no closures) also leaks
3. The whole reason for the closure was to pass in an argument that would allow identification of the widget emitting the signal. This identification can be done with QObject::sender (https://doc.qt.io/qt-6/qobject.html#sender) and avoids the closure.