Highlighting selected cell.

- When the user clicks in a cell, I highlight it in bright blue.
- When user mouses over a cell, I “highlight” it with an inner shadow.
In Chrome, this underlays the cell-selection highlight, so it does what
I need.  If you run into other behavior, please let me know.
- When the user clicks on the highlighted cell, I change the border
color slightly, so it looks correct.  Otherwise, the highlight looked
incongruously bright for the inner shadow I’m currently using.
- When the user clicks in the body, I de-select the current cell, and
update the text-editing area appropriately.