Added a game entity class for refactoring. Player, Obstacle and Background should all subclass this, and should all contain instances of Texture class instead of subclassing them
Added header guard for game entity
incomplete: Refactor variable names from obstacle class and have it subclass from game entity. Still to come: Implement the render method like in player, do not user the Texture class's render method
refactor in progress...
fixed bugs which were introduced by refactoring
fixed the restart issue