Add override for g_binding_group_bind_full()

The function has the same issue as g_object_bind_property_full():

The function itself isn't introspectable because two callbacks share the same closure/destroy.

The binding-friendly _with_closures() variant doesn't work for us either, because we unwrap GValues when passing them to javascript, and GClosures have no way to specify annotations (like "(out)").

Address this the same way as g_object_bind_property_full(), with a small wrapper and a custom override.