Game Restart and Circular Imports

[007vedant]

1. Game music continues even after ESC is pushed and the menu appears again. Music must be restarted.
2. Resolve circular import issues
   1. root should not import main, the restart (ESC) logic should be implemented in game-loop of main.py itself so modify root.py accordingly so that it does not contain import of game_menu() of main.py
   2. Use quit() instead of sys.exit()
   3. Remove unsused variables.