Turn off certain cel's

[Mettra]

Add the ability to not include certain cel's at all for a song. Probably can be done simply by breaking the link in the AssetRoot, although re-finding it a little more tricky.

[DragRedSim]

For the files in the base game that are missing parts, they have the missing part replaced with a "null" object in the .uasset file, still ordered in beat/bass/loop/lead order. I'm thinking that will be all that's needed, since the game needs to start with that file to associate any individual tracks with a song.