binding: Add a default marshaller for the closure API

binding: Add a default marshaller for the closure API

The g_object_bind_property_with_closures() function should set a
marshaller if the two GClosures don't have one already. This simplifies
the caller code and avoids duplication.

We need to add a new marshaller to the gmarshal.list matching the
signature of the GBindingTransformFunc function.