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

> I don't like boolean parameters; what does it mean?

Why? :) I think ```toggle``` pattern is widely known when we can have only ```true``` or ```false``` parameters, like ```classList.toggle(.., true\false)```

> is it a "showSubject" ?

Sure, I can add ```showSubject\hideSubject``` to make it clearer