Separate backpacks into left and right unit#728

Often when creating wings and such, the amount of custom pieces we can slot onto backpacks is too little.

I think separating the backpacks into a left and right module would be a relatively easy way to increase the amounts while also making the mirroring process easier.

Doing it this way could also open the door to some sort of small animation. (angle up/down when boosting, etc)