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`
Previously, in the rare case the database became out of sync with the disk store, it was impossible to feasibly repair a beatmap. Now reimporting checks each file exists on disk and adds it back if it doesn't.