Add graph event listeners

With this patch you can add event listeners to a graph:

* Port added to a graph's component. Note that this is only called when
  the component belongs to a graph. It is not called for existing ports
  when the component becomes part of a graph (with bt_graph_connect()).
* Port removed from a graph's component.
* Graph component's port connected.
* Graph component's port disconnected.

All those listeners return `void` because they cannot "fail". In the
worst case they should just abort. Another way would be to set a flag
at some place and read it once you have called bt_graph_run() or
bt_graph_consume().

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>