If we want to inspect the type of layout properties exposed by a
GtkLayoutManager, we need a way to connect the layout manager type to
the GtkLayoutChild type it creates. In order to do so, we can set the
GtkLayoutChild type on a field of the GtkLayoutManagerClass structure.
Storing the GtkLayoutChild type on the class structure of the layout
manager also allows us to implement a default create_layout_child()
virtual function.
