Bug 944249 - move the segment size and send button management to Compose

that's what I did at first, but then we need to update the segment info when we switch from mms to sms, and so there will be a small gap where we have no info (switch mms to sms => show the counter => the counter is wrong at that moment).

The same happens when the user deletes the last attachment. However, if we're in a case where we have too many segments (very long text), this can be strange.

I can try and see how it looks on the device. I just remember I did this for a reason that I don't remember very well right now ;)