Implement Main Menu and Game Over screens

[WestonVincze]

Map "escape" to pause the game

create a pause menu with an option to return to main menu
optionally auto-pause when not focused on the window
pause menu should overlay with the highest priority and should only pause if the game is not already paused

Ensure the pause system can be easily toggled and modified for the multiplayer version

A similar Menu should be present when the game ends