(Improvement) Add support for NOT possible numbers

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Constant 

What is the expected output? What do you see instead?
I'd like to see possible numbers in the default black, and impossible numbers 
in another color (probably red).

What version of the product are you using? On what operating system?
App version = 1.8r70
OS = Cyanogenmod 9 (Custom ICS)

Please provide any additional information below.
Would require maybe to change from checkbox to 3 choice radio button or combo 
box (maybe, not possible, answer). Alternatively, input could cycle between 
maybe, not possible, answer, blank (removes maybe, like second tap does 
currently) with each press of a number button. 

Original issue reported on code.google.com by dstarf...@gmail.com on 11 Aug 2012 at 8:02

[GoogleCodeExporter]

Original comment by paul.din...@gmail.com on 15 Mar 2013 at 4:01

• Added labels: Type-Enhancement, Usability
• Removed labels: Type-Defect

[GoogleCodeExporter]

Original comment by paul.din...@gmail.com on 21 Mar 2013 at 10:17

• Added labels: Milestone-Infinity