Major Feature Update

• Added 4 Difficulty Levels : Beginner, Easy, Medium and Hard
  • Default at first run is 'Easy'
  • Current Difficulty level is highlighted in blue color
• Changed the method to input numbers in the grid
  • Clicking a box now displays a dialog with buttons 1-9
• Added Dark theme with toggle to switch
  • Default at first run is Dark theme
  • FAB labels don't support theming yet, will be added if (fysoul17/flutter_speed_dial_material_design#7) is merged
• Saves difficulty level and theme locally for subsequent runs
  • Saves it automatically every time you change it
• Animated all Alert Dialogs
• FAB is now disabled until an Alert Dialog is dismissed
• Fixed Alignment of Numbers in the Grid Buttons