pepper: Expose event listener structure and some cleanups

pepper: Expose event listener structure and some cleanups

Event listeners might be frequently inserted/removed. Allocating memory for
each operation might be expensive. Exposing the structure can save such memory
allocations.

* Usage
    pepper_event_listener_init(&listener);
    listener.data = &my_data;
    listener.callback = foo;
    pepper_event_listener_insert(&listener, object, id, priority);

    ...

    pepper_event_listener_remove(&listener);

Change-Id: I1868eae03c0943ee641cade3ace54440ac3f963b