Improving the code of the GP end Cutscenes

My plan was to add a button where you can save a random GP if you liked playing it, but I finally took the redundant code from the two possible GP end scenes and gave it to a new base class. I also removed many unnecessary lines and added a deactivated save button which wouldn't do anything. I'll finish the button in another pull request.
If that was your goal, then I think you got it the wrong way around. saveGPButton performs a hide(), not a show().

So your code actually ensures that "save GP" will only ever appear along "challenge complete" if I understand correctly. In any case that's what I saw when I tested