Fix sudoku saving

Ensure that Sudoku puzzles are saved with the correct type,
and reloaded puzzles are loaded into the correct editor controller
class.
Fix type on SudokuGridViewController loading

Fix a type causing SudokuGridViewController to load
all puzzle cells at index 0, instead of their correct
location

Also cleanup a half-deleted comment that no longer belongs
in this class.
Remove unnecessary comment from KakuroGridViewController

Clean up a half deleted comment that not longer belongs in
KakuroGridViewController.
Add box lines to the sudoku grid.
Create a SudokuGivenCellView

Create a GridCell subclass for sudoku given cells.
It draws its solution in bold, and does not have support
for candidates. Users can toggle betwene given and answer
cells with the enter key or double clicks.