Mixin class for handling widget containers and gui_id

All container widgets (BECFigure and the ones in preparation BECDockArea and BECDock) could benefit from generalised functions from BECFigure:

    def _generate_unique_widget_id(self):
        """Generate a unique widget ID."""
        existing_ids = set(self._widgets.keys())
        for i in itertools.count(1):
            widget_id = f"widget_{i}"
            if widget_id not in existing_ids:
                return widget_id