Bug 906581 - [Messages] Keyboard should always close when pressing Send r=julienw,rwaldron

https://bugzilla.mozilla.org/show_bug.cgi?id=906581

When changing the visible state:

* Don't execute transitionend handler for recipient list visibility unless app is in "new" view
* Don't manually refocus
* Adds conditional guards to avoid TypeErrors when placeholders are null
* Don't remove placeholders that have text
* Do the focus after removing all placeholders, and...
* When focussing in the recipients view, add a new placeholder in case we don't
  have one, and it's the one we want to focus
* Tests
    - Switching multiline to singleline
    - Switching multiline to singleline + refocus
    - Switching singleline to multiline
    - focus without a placeholder