basic, first-iteration inventory management

current issues: player inventory stores items, while the containers
store item-ids, which makes for ugly code. other issue, inventory input
should really be (at least) two states. maybe not InputStates, but some
kinds of states so we can switch 'em (currently have bool _inserting).
allow wielding/wearing/dropping etc. from inventory (but not from within
bags). make inv. management take time (how do we indicate player getting
attacked in background, etc.? close inventory?)