In Detail:
It tried to start a track if the current one ended even if no BeatmapSets were present.
Also if only one BeatmapSet is present if will loop by itself now.
I'm done experimenting, sorry
- `Decoder` only returns a "Beatmap"`Decoder` now
- "Storyboard"`Decoder` is retrieved from a "Beatmap"`Decoder`
- moved parse methods down in the hierarchy where I forgot to do that
- renamed `OsuLegacyDecoderTest` to `LegacyDecoderTest`