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

TBH I'm not sure this piece of code about updateHeaderData is still useful. At one point we were re-rendering the threadlist each time there was a change (like sending a message, etc), and maybe this was used at this moment.

But this is not the case now so I wonder if we can not just remove it. (I'm not talking about the Threads.set call above, but the updateHeaderData below)