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.
I'm not sure, I'm easy if you think that's better - I specifically check for Divider/Placeholder because those are the items we remove before rendering - are we likely to start removing other types of item before rendering?