Restart button added!!!

Now we have a way to retry the stage when you fail or pass the game. Just press R on your keyboard and everything is restarted.

There is something I've learned about programming during making this update. Trying to implement a restart function to the custom-made engine forced me to look deep into the scene-loading procedure: how every loaded resources can be reused? As I simply splitted the loading procedure into two stages -- one for initialization, one for beginning reset -- I found out that reusing some of the resources may lead to memory leak or crashes, as the objects of certain resource classes are simply not designed for multiple uses. Now everything is as good as wished, except for that the audio-timer would pause for ~300ms right after a restart, which doesn't really matters as the actual game starts after a whole second.

Hope that I can make more stages and add a formal menu screen for those stages.