st-box-layout: Avoid fullscreen relayout on scroll

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.