Improve key controls, add better marking support

[EmilyV99]

• "Answer" numbers and "marking" numbers are now handled separately, allowing you to enter markings and then type an answer OVER them.
  • The markings will now always appear in numerical order
  • As such, markings now require a separate entry mode- selected via radiobutton, toggled via tab, or temporarily toggled while holding Shift.
  • This DOES override Tab's normal feature of cycling through selected objects... if that's an issue, the tab toggle can be changed to some other key.
• Arrow keys now navigate the grid properly
• Backspace/Delete now will delete all markings instead of the rightmost marking; or, if an answer is placed in a cell, it will delete the answer (without deleting markings that were hiding underneath the answer).
• Changed the text color of user-entered numbers (Both markings AND answers) to better differentiate user-entered numbers from given digits. These colors are now static near the top of SudokuCell.cs for ease-of-changing.

Overall, these changes attempt to bring this more in line with SudokuPad used by Cracking the Cryptic. (More ideas from this that would be useful would include multi-highlighting to enter into multiple cells at once, and a third entry mode for corner-marking, though the former is a bit beyond me, and the latter I'm not sure how to go about displaying the numbers properly - would probably require a fair bit of work, and new UI elements on each cell...)

This fixes #3, and makes #5 and #8 redundant.

[Jarno458]

project is nolonger maintained