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

ok, now that I tried, I see what does not work. Here is the failing STR:
1. write a long enough message to trigger MMS with only text (you can change `Settings.maxConcatenatedMessages` to try this quickly :) )
2. then remove enough characters to trigger SMS

If we don't get the segment info while we're in MMS mode, we're never back to SMS mode.

So... what do you think ? :)