Bug 1025565 - Only render the necessary icons when dragging. r=kgrandon

Dragging an icon previously caused all icons from the first icon that needs
re-rendering to the end of the grid. This makes sure that only the icons
that are affected by the rearrangement are re-rendered.
nit: i wonder if we should check on item.identifier instead. Potentially more future-proof and shorter. What do you think?