problem here is that on https://github.com/fcampo/gaia/blob/268092114029d743fb062c3f1b46aef64829d83d/apps/sms/js/compose.js#L85 we have already a function called isShowing (so we are able to use `Compose.subject.isShowing`). If we use the same name for the flag, we will have trouble.

I'm ok with the change of names to make it clearer, but in this case we should give it a spin. 
Maybe ìsVisible`?