Add frozen property to notification objects

A frozen notification cannot be modified (except for its reference
count). Currently, no notification can be modified after its creation
anyway, but we're excepting some.

The notification is frozen by bt_notification_iterator_next() when the
component's "next" method is successful.

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