create introspection-friendly version of g_settings_bind_with_mapping
Submitted by Evan Nemerson
Link to original bug (#678841)
Description
g_settings_bind_with_mapping is currently annotated with (skip) because of the shared user_data argument, which GIR doesn't support (and neither does Vala, FWIW). AFAICT there is no straightforward introspection/vala-friendly way to access this functionality, so it would be nice if GIO provided a version of the function which provided one user data and one destroy notify per callback.