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

My only fear about this is in the order of listener: if ThreadUI's listener runs before ThreadListUI's listener, then the message won't be registered when ThreadUI's listener runs, and we can have unintended behaviors.

I'm not saying it's the case now, but this can definitely happen in the future and this can give weird difficult-to-debug bugs.

So I'd be more conservative and keep the Threads.registerMessage call here.