Add various events useful for third party integrations.

Adds event triggers for "on notebook list loaded", "on cell executed", and "on item deleted".  These were events that we found useful for analytics and other custom logic in our extensions.
Sure.  For my own edification, what's the benefit of passing as an object rather than an array?  I've seen jquery triggers elsewhere that pass additional parameters like `trigger('execute.CodeCell', [cell])`.