Remove the item limit

This PR does the following:
1) Add support for using position in inventory & player, and the ability to move between position&invlet.  Position is the index into the player's inventory if positive, -1 for weapons, and -2 or lower for worn items (-2-worn[index]).
2) Make inventory uis return position and allow selecting items without an invlet.
3) Move users of inventory to use position instead of invlet
4) Remove existing item-limit checks, but add new checks that expect/assign invlets on wielded/worn items.

It would have been nice to add the ability to permanently clear an assigned invlet w/ spacebar, but the restacking code is aggressive about giving invlets and quickly undoes anything the player does there.