Bug 1073807 - [2.0 only] Improve smoothness of utility tray

I'm not a fan of having 'safety timeouts' - bugs that stop transitions from happening have been fixed in platform, we can craft our code so as to not need them now (and though I wouldn't r- over it, I think we should to reduce overall complexity.

If you find there are situations where transitionend isn't firing, you can access a dom layout property (such as clientLeft) to force a flush before setting the appropriate class. While that's also a hack, it's a one line hack that's easily commented, and the possible regression from removing it is easily tested.