Hook up color printing with print dialog.

- Add a button and callback to the print dialog to let the user select
  color printing.
- Add a state variable to the options struct to track the users choice.
- Use a darker color for the grid on dive plot; that way we can see it.
- Default to use color printing.

Signed-off-by: Amit Chaudhuri <amit.k.chaudhuri@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>