Bug 1061215 - [Messages][Refactoring] Improve handling of remaining chars counter. r=schung

actually, in my head, we want to eventually separate Compose in 2 parts: one part that would be the controller, and one part that would be the renderer (or the view).

That's why I think we should listen internally to the segmentinfochange event, and call this method as handler. What do you think?