Add a clear all notifications effect

bugzilla bug 
https://bugzilla.mozilla.org/show_bug.cgi?id=1092536 

I implement a simple slide to right animation,
I changed the ns_closeNotification in a backward compatible way, so that when no delay are passed it will just work as before (if you try to swipe for example the new effect doesn't collide).
On the other hand you can now pass a delay and a waitBeforeClosing parameter, and if you do, there will be a percentual TranslateX applied to each notification.

The waitBeforeClosing is needed to avoid the animation that will trigger when a single notification get removed.
Another solution would have been to start from the bottom, but that's just does not feel natural.

I think this setTimeout call could be replaced with a transition delay (so 'last.style.transitionDelay = this.CLEAR_DELAY * index++; last.classList.add('disappearing-with-not-push-up'); last.style.transform = '';') - As the notification gets destroyed after clearing, I assume we wouldn't need to worry about cleaning up the set transition-delay?

The rationale for using transition-delay rather than setTimeout is that setTimeout doesn't have any hard guarantees about it actually happening when you tell it to, so the animation could look uneven if the phone was busy - the transition-delay will be sent to the compositor with the rest of the animation detail, so it gets what for all intents and purposes ends up being a hard guarantee over its start-time, at least relative to the rest of the animation.