Registers an EventListenerCB against an event_type.
| callback |
the EventListenerCB to be registered against an event type. |
| event_type |
a character string indicating the type of events for which notification is requested. See register for a description of the format. |
| callback_destroyed |
A DestroyNotify called when the callback is destroyed. |
| user_data |
User data to be passed to the callback. |
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