Don't display 'no item' response if another PC can shop.

[NQNStudios]

In comments on #669 I outlined a way to intuitively handle shopping from a healer if the active PC is fully healed. This PR implements my suggestion. I think it feels pretty good and makes shopping better even outside the case that was buggy before: it's just nice to have the shop cycle between injured PCs if you're trying to buy healing for everyone.

These are the rules:

• If the active PC gets an empty shop, iterate from the first to last PC until finding someone who can see a shop item
  • If none can shop, show the 'no item' response
• When the active PC buys something, if this clears out the shop, iterate from the first to last PC until finding someone who can see a shop item
  • if none can see an item, stay on the empty shop of the PC that was active when the last item was bought.
• When shopping ends, if the shop interface was the last thing to change the player's active PC, it puts it back how it was.

Fix #669