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

I think it would be better to subscribe in ```beforeEnter``` and unsubsctibe in ```afterLeave``` once we split Thread and Composer panels. Less checks on current panel and so on. At least Composer doesn't need following events for sure: ```onMessageSent```, ```onMessageReceived```, ```onMessageDelivered``` and ```onMessageRead```. Not sure about ```onMessageFailedToSend```, do you know if ```failed``` can be called before or instead of ```sending```?