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
state to Paused or Suspended also sets currentScene.running to false.