Bug 1022713 - Only render the necessary icons when dragging. r=

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.
Bug 1022713 - Fix the possibility of picking lower res icons than needed. r=
Bug 1022713 - Separate grid icon into a separate element. r=
Bug 1022713 - Remove 'dragging' class on transitionend to allow icon drop transitions. r=
Bug 1022713 - Add styles for when the grid is scrolling and for visible items. r=