Bug 885283 - [sms] receiving a SMS from an existing thread but on a new thread container is broken r=gnarf

* Refactored how threads are added when a new message is received
* This incidentally resolves the bug
* made FixedHeader's refresh and scroll handler so that it's being called once
  per tick only. Then we won't be afraid of calling refresh too many.
* remove some unused code in FixedHeader
* adds tests for FixedHeader
* Fix indentation of createThreadMockup