Widget drop targets respond to location of cursor. Hilite drop target when targeted.

visually exposes issue with widget drop-targets

setting a background on the draggable, and setting a hover state on droppable makes it clear why widgets are getting added to multiple areas.

![screen shot 2014-06-27 at 12 35 14 pm](https://cloud.githubusercontent.com/assets/6045075/3416928/53bde7aa-fe32-11e3-97ad-8472a7e05f21.png)