This custom QLayout class allows for overlaying child widgets on top
of their parent widget.
It can be used like this:
layout = QgsOverlayWidgetLayout()
parent_widget.setLayout(layout)
layout.setContentsMargins(20,20,20,20)
layout.addWidget(QGroupBox(), Qt.Edge.LeftEdge)
layout.addWidget(QLabel('My label on top of a parent!'), Qt.Edge.TopEdge)
layout.addWidget(QGroupBox(), Qt.Edge.TopEdge)
