Event notification: make sure contained event has a trace

Event notification: make sure contained event has a trace

Because we need to keep support for legacy CTF writer behaviours, we can
still create a "floating" event without its stream class attached to a
trace. Disallow the creation of an event notification with such an event
to provide the guarantee to filter and sink components that you can
always find the trace of an event notification's event.

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