bitbake: runqueue: add sceneQueueTaskCompleted event

Adding an event to be fired when a scene task is completed.
It is analogous to the run task completed event, and has
been missing for some reason.

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>