Identifiables

[jedediah]

• Extract common interface Identifiable for things identified by a UUID, i.e. Entitys and OfflinePlayers.
• Add ConnectedPlayer interface between OfflinePlayer and Player in the inheritance hiearchy, which adds a field for IP address. Use this to carry info in pre-login events.
• Add Event#getInvolved() that returns a list of all Identifiables involved in the event.

This seems like a better approach than EntityAction and PlayerAction, and we may remove that patch if this one serves our needs.