Split out legacy model export logic into `LegacyModelExporter` classes
Remove pointless interface class for `IBeatmapModelManager`
IBeatmapModelManager
Was originally going to be used but serves no purpose any more.
Initialise `BeatmapSet.Beatmaps` list at construction time
Beatmap
BeatmapSetInfo
BeatmapSet.Beatmaps
StoragePath
BeatmapSetInfo.OnlineBeatmapSetID
OnlineID
BeatmapInfo.OnlineBeatmapID
BeatmapInfo.Version
DifficultyName
BeatmapInfo.StarDifficulty
StarRating
BeatmapInfo
PlayableBeatmap.BeatmapInfo
BeatmapModelManager.Save
IModelDownloader
ScoreManager
BeatmapManager