Add parameters to callbacks

This patchs adds a new structure that is passed by the library as the
first parameter of each callback call. As of now it only provides a
pointer to the current event. Now we also pass the pointer to the
private data registered when the callback is added.

Signed-off-by: Julien Desfossez <julien.desfossez@polymtl.ca>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>