Main Menu, Basic UI Elements, and GameManger Improvements

[SonicKurt]

I have created the basic layout for the main menu. You can either click on the "1 Player" or "2 Players" buttons or press the "1" or "2" keys to start playing a one or two-player game. It will initially display the score text for every scene throughout the game. Once you start a game, it will initially display the current player and stage. After a couple of seconds, it will start loading in the aliens. I have made a MenuManager to keep track of the current elements that need to display for the user within the particular game state.

I also implemented a simple lives counter for the player to start off with the value from the initialLives variable.

Please let me know if you all have any questions or need additional changes for a future issue. Thanks.

[SonicKurt]

Design: Menu Schema

[SonicKurt]

Current main menu state.