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

> we can also bind onSegmentInfoChange on the "type" event; should be similar to previous solution, if I don't miss something?

Yeah

>most correct solution IMO: ** when going from SMS to MMS => empty the counter ** when going from MMS to SMS => wait for the segmentinfo change to add the counter back (means, don't change this code) ** if I don't miss something, it means we'll have the new computed correct counter count after a roundtrip with the gecko API, right?

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)