Implement a simple priority system.

The priority system is used to determine which screen is shown first if two or more screens
have updated information (ie. a group screen is more important than a "today's high scores"
screen, which is in turn more important than "today's most chosen songs", as the latter is
mainly a curiosity).

- Add a get_priority() to GenericScreen, default 0.
- Implement get_priority() for GroupScreen and Top10ScoreScreen, set to 10 and 5 resp.
- Make RotateScreen automatically pick the screen with the highest priority if two or more
  have forced updates.