reverted split at Decoder, moved logic down

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`

osu.Game/Tests/Visual/TestCasePlayer.cs

@@ -63,7 +63,7 @@ namespace osu.Game.Tests.Visual
 
             using (var stream = new MemoryStream(Encoding.UTF8.GetBytes(test_beatmap_data)))
             using (var reader = new StreamReader(stream))
-                beatmap = Game.Beatmaps.Formats.Decoder.GetBeatmapDecoder(reader).DecodeBeatmap(reader);
+                beatmap = Game.Beatmaps.Formats.Decoder.GetDecoder(reader).DecodeBeatmap(reader);
 
             return beatmap;
         }