Refactor the folder structure in the skinning folder.

[andy840119]

What's the goal of this discussion:

1. Make the folder structure look similar to other rulesets.
2. Remove some weird classes that should not appear in ruleset (e.g. KaraokeSkin)
3. Move some class out of root folder of skinning ruleset (e.g. BitmapFontCompressor)

Skinnable 和其他物件 (e.g. ruleset config, beatmap metadata) 之間的關係可以參考:

• https://karaoke-dev.notion.site/Configuration-4213504e89a74a9f93f44b742e2f0e92?pvs=4