Makes `Trade.MyOfferedItems` public and fixes `Bot.OtherInventory` to work with multiple users/bots.

This pull request makes Trade.MyOfferedItems public so you can keep track of what items the bot has added without creating your own lists, which will make counting items added more reliable.

Additionally, I've fixed Bot.OtherInventory so that it will work with multiple users. This will require a small code change to UserHandlers: where you previously accessed the inventory with `Bot.OtherInventory` you will now need an additional parameter, e.g. `Bot.OtherInventory(OtherSID)`. `Bot.GetInventory` will continue to work as normal. This pull request fixes #566 and #543.