Enhancement: Full Multikey Support

[Vernoc]

I found it weird that while you can freely change the key count of strumlines, there was no way to play charts with these key counts and there were only skins for 4 of the notes.

It would be helpful for creators to have these changes without softcoding or hardcoding the engine to have these features. (also shaggy mod ports)

[r6915ee]

There is #996 where some things to do to implement this in softcoded mods are discussed. That being said, I haven't heard of any idea to implement support for this in Codename Engine itself.