Graph: pass remove listeners when adding listeners

Graph: pass remove listeners when adding listeners

With this patch, when you call bt_graph_add_*_listener(), you can also
provide a "remove listener" which is called when the added listener is
removed when the graph object is destroyed. This is useful when you
dynamically allocate data only for the listener to add and you are not
the only owner of the graph object: the remove listener is where to
free/release this data.

Existing CLIs and tests are modified to match this change.

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