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=
Bug XXX - Add an alternative abspos item renderer for better performance. r=

Absolute positioning allows more aggressive layer flattening than using
transforms, so use it to open up the posssibility of animating grid icons
without causing layerisation of grid item text.
Bug 1009530 - Add visual feedback for edit mode. r=

Add visual feedback when in edit mode and make various changes to
maintain performance.