[WIP] Interruptable drop and pickup

This makes dropping and picking things up into an activity, which means it can be interrupted like any other activity by e.g. seeing a monster approach or taking damage.
- [x] drop activity
- [x] stash item in pet inventory activity
- [ ] pickup activity
- [ ] advanced inventory
  - [ ] make advanced inventory drop items immediately instead of queuing them to move
  - [ ] make advanced inventory kick you out of the menu when you run out of moves, then re-enter the menu seamlessly
  - [ ] hook up move all to start an appropriate drop or pickup activity, then re-enter the menu when it's done.