Bug 1041124 - [Messages][Refactoring] Make use of EventDispatcher in MessageManager. r=julien

It really doesn't matter for me where to call this, the only thing that I want to do is to unsubscribe as soon as possible :) Probably it's only for me, but it's difficult to understand\remember sometime whether panel should listen for something when it's not active (like in always-in-place ThreadListUI) or not. But when I see that panel is unsubsribing from something as soon as it becomes inactive then it's more clear to me. So I'm glad that you don't block on this, it doesn't bring much complexity and at the same time makes me feel more safe :D