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.

If there's nothing clearable, we can just return I would suppose? In that case, let's change this to:

this.clearAllButton.disabled = true;

if (!clearable.length) {
  return;
}

And move the logic that was inside this block to after that.