Hopefully allows things in inventories to see emotes.

Opening this to dev-freeze in the perhaps vain hope that it will get merged. It's a fix-bug-feature-thing, really.

Haven't had much of a chance to test this yet since it requires a minimum of 3 keys.

- Adds a PASSEMOTES flag.
- Changes borers and dionaea to set this flag.
- Uses this flag in custom_emote() to pass emotes to holders and borers in a mob's inventory.