adds instance variables to keep track of main menu vs. difficulty menu
adds instance variables to keep track of main menu vs. difficulty menu

ruslan120101/TankAttack-JavaFX,ruslan120101/JavaFX-TankAttack,ruslan120101/JavaFX-TankAttack,ruslan120101/JavaFX-TankAttack,ruslan120101/TankAttack-JavaFX,ruslan120101/TankAttack-JavaFX