cleaned up the code a lot

- changed the variables from public to private in Entity class
- created setters and getters in Entity class
- added the singleton pattern in the player class so that only one
player can ever be instasiated except when loading
- changed accessing public variables of Entity to accessing them
through accessor "get" methods.
- Instead of player.backpack it's player.getBackpack()
-cleaned up code in all the menus (PlayerMenu, DebugMenu...)