Refuse to add port to component when parent graph is canceled

The objects owned by a graph should not be able to perform any useless
action. Having those constraints promise that certain contained objects
remain within certain states when the graph is canceled.

In this case, it is useless for a component to add a port, since this
port cannot be connected to another one within the same graph because
bt_graph_connect_ports() is not permitted when the graph is canceled.

We still allow ports to be removed when the graph is canceled for
symetric teardowns, for example.

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