Previously StBoxLayout (e.g. the icon grid) would scroll by queuing a
relayout, which trickled up to the stage and would relayout the entire
scene. That was a very expensive thing to do for each 1-pixel scroll event.
We now avoid the expensive clutter_actor_queue_relayout route and instead
only queue redraws on the actor hierarchy being scrolled. Although the
redraw also trickles up to the stage it's cheaper than a relayout. This
makes scrolling the icon grid much smoother and consistently maintains
60 FPS where previously it would often be 30 FPS.