Add add_event_listener() to VAPI
Correct VAPI for add_event_listener_with_closure()

Based on Evan Nemerson's commit to Vala:
https://git.gnome.org/browse/vala/commit/?id=6a1a1ab1637455f43e5e8dd707da1bb3bec37f66

Also, remove previous changes to VAPI, since this is more convenient
than the add_event_listener() function.  (Instance methods are
acceptible.)