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

> Or we may want to separate in 2 elements

IMO it's the only viable alternative...

>Also, is it possible to use classes instead of ids here?

Yes, we can and should, I'm doing it in the next patch for the top part, and reduce ids for the bottom part in the next one (making Compose standalone component) :) If you don't mind :)