Skinning: Use separate soyokaze/ folder for skin elements

[goodtrailer]

Fixes #168.
Fixes #223.

Copied from README.md, which has been updated accordingly:

By default, skin PNGs will be taken from the skin's root folder (i.e. where stable looks for skin PNGs). So certain elements (hit circles, approach circles, judgements) will share the same PNGs as osu!std. However, soyokaze! will also look for skin PNGs inside a separate soyokaze/ folder (i.e. soyokaze/hitcircle@2x.png instead of hitcircle@2x.png), so it is possible to differentiate between soyokaze! and osu!std. For better organization, all soyokaze! PNGs can be placed inside this soyokaze/ folder.

Note that hit circle text is not supported. There doesn't seem to be any easy way to add additional text types, since most of this is implemented via static methods with no immediate extensibility.

[goodtrailer]

CodeFactor is so dumb sometimes.