general refactoring

- changed while loop in Menu() (Game.cs) to do-while - more appropriate
- renamed 'str' to 'input' in Menu()
- extracted logic for playing move to PlayMove() method
- extracted logic for saving highscores to SaveHighScore() method
- extracted logic for processing commands to ProcessCommand() method
- merged case for winning the game with the case of losing (most of the
code is repeated, except for the printed string)(from PlayMove() method)
- removed Field wrapper class and left FieldCell class only (redundant)
- renamed FieldStatus to CellStatus (to keep things consistent)