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.
This C++ 2011 usage should be avoided, I have been working hard to remove all places where it was used, since it doesn't compile on my oldish mac. Please do not introduce new ones!