WIP: st-icon: Avoid reloading icons during animations [performance]

When gnome-shell is animating we get constant style-changed callbacks but for icons flying around that only means the scale, position and opacity is changing. It's still the same icon. So avoid reloading icons on every frame if they're really the same icons. This also avoids triggering clutter_actor_queue_relayout which would trigger a very expensive full stage relayout.

Related to mutter#232 (closed) and mutter#233 (closed).