Store item id selected before item disappears from shop stock during sell-x dialogue

[Hubcapp]

We could store the reference to item ID selected when the Sell-X dialogue box is summoned, then just always use that last-selected-item when the packet is sent. This way the user would not have their input discarded.

Related to authentic bug #64, fixed in PR #68.

[lmsv-mx123]

For reference on anyone taking look into enhancement, also should be considered case for Buy-x

[Hubcapp]

Technically Buy-X might be possible to trigger, but the setup for that is much rarer.

1. has to be an item that store doesn't have any base stock of, because otherwise the index won't disappear, the shop just says "has 0"
2. player starts buying "x" of item they would not normally expect in the store
3. all stock is depleted!
4. stock is replenished by a second player in time for the buy-x to make sense again (if step 4 doesn't happen, then current behaviour is fine, because it just exits gracefully)
5. send the buy-x with the last selected item id

Set up for sell-x:

1. sell a few of an item to shop that they don't normally stock
2. realize you actually need to sell quite a few, sell-X
3. shop despawns quick & the item id gets set to -1
4. send the sell-x with the last selected item id