Fixes to animations (transitions) and notifications

Make those animations buttery smooth, glitchless and awesome.

* Put all notifications into a separate #notifications div (helps
  sandbox those transformed elements better, seems to fix blurry text
  for good).
* Create a more universal transition LESS mixin (accepts arbitrary
  number of transitions just like transition CSS property).
* Animate only the required properties for notifications (not all
  properties).
* Provide a reliable fallback for browsers that don't support
  position: fixed (that works both with and without animations).
* Stop treating Firefox and MSIE 10 (Windows Phone 8) as if they didn't
  support position: fixed.

Bug: 50621
Change-Id: I9aa603fae53c36184421c520e911f6e4b9ba5300
Merge "Fixes to animations (transitions) and notifications"