Core/Opcodes: Add some opcodes

CMSG_GET_MAIL_LIST
CMSG_MAIL_MARK_AS_READ
CMSG_MAIL_RETURN_TO_SENDER
CMSG_MAIL_TAKE_ITEM
CMSG_MAIL_TAKE_MONEY
CMSG_SETSHEATHED
SMSG_GAMEOBJECT_QUERY_RESPONSE