Add delegate methods for adjusting drag-drop visuals in custom ways.

I updated the example project to demonstrate this by setting partial alpha and doing a rotate+scale.  

The two new delegate methods add a simple way for consumers to control the visual changes to the draggable view that represents the moving cell.  The first new method allows for custom visual adjustments when the cell is first grabbed, the second allows for undoing the visual adjustments when the cell is released.