clear a cell on backspace rather than show '0'

Thanks Ravi for the idea.

0017-on.keychord_press

@@ -29,8 +29,11 @@ print(chord)
 			-- it's not in the provided square
 			if x ~= Problem.x or y ~= Problem.y then
 				local c = Problem.data[y][x]
-				if c ~= '' and c > 0 then
+				if c ~= '' then
 					Problem.data[y][x] = math.floor(c/10)
+					if Problem.data[y][x] == 0 then
+						Problem.data[y][x] = ''
+					end
 				end
 			end
 		end