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.
I first tried to modify the xml, but there's no way to say that it should be hidden and my own modifications to screen_loader.cpp failed (not sure if this feature should be added). I could change the visibility in the code and move the rest to the xml, but I don't know how to give the "save"-button the same height as the "continue"-button.