Bugfixes and new features:
- Added Diagonal Sudoku support!
- Added Preferences. Users can now disable/enable cell highlighting based on the sudoku variant they're playing.
- Added Casual Mode. When on, entries are checked against the solution. When off, only rule violations are flagged.
- Fixed a bug that made blocks untouchable after completion.
- Updated the About section.
- Note addition popover now stays open unless the user presses the Done button.
