Event handler cleanup

[jng]

In the Maestro codebase there's lots of event subscription with manual cleanup, and there's probably leaks scattered about because we haven't properly unsubscribed from some events.

Using weak event handlers with auto-deregistration is much simpler to manage:

• ​http://stackoverflow.com/questions/1089309/weak-events-in-net
• ​http://diditwith.net/2007/03/23/SolvingTheProblemWithEventsWeakEventHandlers.aspx