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

> Sounds like a good idea, will leave this if and clear counter in onTypeChange. Then user will only see the correct value or no value at all (during segment info request)

Well segmentInfo is called only on content change and isn't called on subject change. It works in master because dom counter is updated all the time (for SMS and MMS). It works correctly, but ineffcient - will try to make it correct once and for all :)