Improve the scoring system

[hvlds]

The actual scoring system is inconsistent and not so clear for new players.
New possible scoring system for p players and be m a position between first and last player:

• last player = 0 points
• first player = p - 1 points
  And therefore in general:
• m-th player = p - m points

The max amount of point can stay the same (p * 5, eg: 2 players -> 10 points), but include a system with min. amount of points of difference between 1st and 2nd place (2, maybe?).

The scoring is calculated and updated in player_manager.cpp.

• Calculation -> PlayerManager::update_score()
• Check if the max. amount was obtained -> PlayerManager::check_score()