Diff game re-write part 1

- Replaced all diff game pictures
- Diff game now only uses one scene
- Diff game uses helper class called DiffPictureObject which contains
  all necessary information for every level. It also looks good in the
  inspector.
- Made GoToNextLevel function overridable in GameManager so
  that one scene machinery can work correctly
- Levels are randomized each time the game is started

Signed-off-by: Vinh Truong <vinh.truong@metropolia.fi>