Fix and refactor star difficulty calculation boilerplate

Moves star difficulty calculation entry-point to Beatmap, and sets
star difficulty at the correct place for song select to display.

osu.Game.Modes.Osu/OsuDifficultyCalculator.cs

@@ -34,7 +34,7 @@ namespace osu.Game.Modes.Osu
                     ((Slider)h).Curve.Calculate();
         }
 
-        protected override double ComputeDifficulty(Dictionary<String, String> categoryDifficulty)
+        protected override double CalculateInternal(Dictionary<String, String> categoryDifficulty)
         {
             // Fill our custom DifficultyHitObject class, that carries additional information
             DifficultyHitObjects.Clear();