Bug 876803 - Thread List UI: Select some threads and press delete - r=schung

The old logic wasn't properly keeping track of variables for delete thread.
Also, we shouldn't hide the WaitingScreen or require a re-render from MessageManager if we fix ThreadListUI.removeThread
Abstracts the adding/removing of empty classes in ThreadListUI
(cherry picked from commit 451796de73fb57368fa419e25efdc86e7ecebb2f)