Add victory and instruction screens

Prior to this commit, it was not clear to the player when they had
beaten the game and there were no instructions for how to play. Now
two new screens have been added, one with instructions and one for
when the player has completed all the levels.
Fix a few bugs in the new display screens, display final score

Fix a few bugs with the screens added in the last commit. Also
update the code so that the final score is displayed on end game
screens.