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

In this current state I think it's appropriate, even that I don't like ```handleEvent``` approach too much because every new update into particular processing code usually leads to a bigger ```handleEvent``` instead of extracting it into separate method. Also not sure whether it's possible to extract event name from even object itself (otherwise we'll need to bind it on declaration stage). + There is a high risk that DS work will change it once again :)

So if you think it's better I can try move it into ```handleEvent``` :)