Connect LayoutManager to LayoutChild
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.