Various fixes

A few fixes I've made to the repo. I think you can pull individual commits that you want, they're pretty separate.

The fixes:

* Blackboard didn't have the bot_start_sneaking method, so I added it.
* ef054ac266 & 13b8e05715 need to be together, 13b8e05715 fixes problems in the first. This fixes the crash when a bot picks up inventory_avoid items.
* The last one is just a tiny fix for XP orbs, since the code assumed they were item stacks. I just had it ignore XP orbs, since to get your real XP you always need to check the server anyways (see commit message)