[WIP][CR]Backpack layer and stuff

I am working on #9821 with some additional tidbits.

New layers for things worn on the back (backpacks mainly, but not exclusively, quivers goes there too) and items worn over the shoulder (fanny pack, messenger bag, purse... submachine gun with sling :wink: ). 

- [x] backpack and over-shoulder layers
- [x] enforce max 1 backpack
- [x] change scabbard to be worn on legs, along sheaths and holsters (and limit max weapon size)
- [x] add large scabbard (worn on torso - backpack layer) for large weapons (nodachi, two-handers ...)
- [ ] balancing encumbrance and storage as deemed necessary, comments welcomed
- [ ] help layers description update
Should probably be "You're already wearing A backpack."