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

Agree that calling it here isn't good! But not sure that I understand why updating type dependent indicator (for MMS it shows label, for SMS it shows counter) is side effect... Currently the best way I can think of is very similar to what we have here (I read it like "update type dependent indicator when type is actually changed"):
```
this.on('type', this.updateMiscellaneous.bind(this));
````

Doesn't it work for you?