The GtkWidget::parent-set signal was removed in ff6cd8f7.
Instead of removing GtkLayoutChild instances associated to a widget
using notifications when the widget's parent changes, we can have
gtk_widget_unparent() call a method on GtkLayoutManager to remove any
eventual GtkLayoutChild instances associated to the widget.
A base abstract class for layout manager delegate objects.
Layout managers are associated to a single widget, like event
controllers, and are responsible for measuring and allocating the
children of the widget they are bound to.