Bug 1048845 - [Messages] Compose Panel refresh (subject behavior change). r=schung

Totally agree with you on this, one quick idea that I have in mind is to get rid of ```:after``` and ```data-counter``` at all and use ```textContent``` (via ```setAttribute('data-l10n-id', ...) for MMS case) instead on the same element. I'll try to do this in this patch, but if it requires more changes that I expected I'll move it to separate bug.