Add a convenience api to skip children
that should not be included in the layout,
and call gtk_native_check_resize on all
native children outside of the vfunc.
Make the transform (transfer full).
1. This makes sure we actually reference the transform. Previously we
did not.
2. Most callers create a new transform to pass to us. Now they don't
have to uref it anymore.
Just like GtkContainer provides a default implementation of
GtkWidgetClass.get_request_mode(), we can do the same inside
GtkLayoutManager.
A default implementation preserves the behavior of existing widgets that
moved, or will move, to a GtkLayoutManager.
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.
