layout: Improve rendering performance and clean up CSS

* Resolve annoying conflict with sidebar transition selectors
  by removing the vector-hd parent class. These rules are
  useless anyway in this case as the sidebar is hidden in RTRC.

* Optimise page rendering by transitioning the the sidebar,
  navigation and content all at the same time.

* Using translate() for the sidebar transition.

* Keeping page content hidden until page transformation is
  complete, this avoids the page contents from having to redraw
  a million times during first ~ 250ms because the width is
  still changing.

* Moving code for namespace tabs change from the general init to
  a dedicated $() handler that doesn't have to wait for all the
  other deferreds to finish first (localisation, modules etc.)
  This makes the unchanged namespace tabs essentially never visible
  in the first place as we're changing them straight from dom-ready.
  Fixes the visible switch of tabs during the startup.

* Remove odd 'text-align: left' on .switch+div:before,
  the container should have text-align instead.
  The heads in the mini group panels don't need the inherited
  text-align: center, and the regular ones already inherit it.