Added Bacon2D::State enum and gameState property to Game. 

Added Bacon2D::State enum and gameState property to Game.  Pausing the game should be done by setting Game.gameState to Bacon2D.Paused.  This also listens to Qt.ApplicationState changes and sets Game.gameState to Bacon2D.Suspended if Qt.ApplicationState isn't active and the game isn't paused.  Setting gameState to Paused or Suspended also sets currentScene.running to false.
property is not read-only, so the user can change value. But also Game::onApplicationStateChanged change it's value. I think it should be read-only, right?