Migrate osu to use the new difficulty calculator structure
Fix TestCaseMultiScreen intermittent failures
# Conflicts: # osu.Game.Rulesets.Catch/Difficulty/CatchLegacyDifficultyCalculator.cs
# Conflicts: # osu.Game.Rulesets.Taiko/Difficulty/TaikoLegacyDifficultyCalculator.cs
# Conflicts: # osu.Game.Rulesets.Mania/Difficulty/ManiaLegacyDifficultyCalculator.cs
Implement new difficulty calculator structure
# Conflicts: # osu.Game.Rulesets.Osu/Difficulty/OsuLegacyDifficultyCalculator.cs