Added Game::State enum and state property to Game. 

Added Game::State enum and state property to Game.  Pausing the game should be done by setting Game.state to Game.Paused.  This also listens to Qt.ApplicationState changes and sets Game.state to Game.Suspended if
Qt.ApplicationState isn't active and the game isn't paused.  Setting state to Paused or Suspended also sets currentScene.running to false.