|SOURCE|
[Common::GameStateManager]
* Added a private member function, IsGameStateNameValid, which iterates over
  the registry of game states, checking if the game state name passed in
  matches any in the registry, returning ZED_FALSE if nothing was found,
  ZED_TRUE, otherwise
* ChangeState pops the top state off the stack and replaces it with the
  requested one if it is valid
* PushState determines if a state's name is valid, if it is then its Enter
  function is called, if that fails, the funciton pops the top game
  state off the stack and returns ZED_FAIL, otherwise it will return ZED_OK
* PopState pops the top-most state off the stack