Added GameState and GameStateManager, changed the clear colour and Makefile

|BUILD CONFIGURATION|
[Linux]
* Added the XInput library to the list of libraries to link against

|SOURCE|
[Common/GameState]
* This class is responsible for creating different game state types, such as
  menu state and gameplay state
* This class is intended to be used as a base for concrete classes
[Common/GameStateManager]
* The GameStateManager maintains the game states via a stack
* States can be pushed onto or popped off of the stack