Implemented a screen component

Moved SDL initialization into cGame_Manager.initializeSDL
Added a create_screen_entity method to cGame_Manager.
create_screen_entity and create_user_entity both register the created
entity immediately now
Removed screen initialization from main.cpp to cGame_Manager
Modified drawing code in main.cpp to use the created screen entity
Added a const int SCREEN = -1 to act as the entity id for the screen
enitity