Bug 840069 [Messaging:MMS] Expose .on to attach handlers to Compose acti...

...ons, like 'input'. Expose getText() to get text content. Update ThreadUI to use Compose.getText() and Compose.setMaxLength() as well as expose state of empty/non-empty via isEmpty()

the changes that @borjasalguero wanted from https://github.com/mozilla-b2g/gaia/pull/9385/