Lua: Added ProtoField.none

This field can be used for adding filterable subtree entries.

Change-Id: Id395c9a8194dc00e6544170631b48d71ae9dd33e
Reviewed-on: https://code.wireshark.org/review/4573
Reviewed-by: Evan Huus <eapache@gmail.com>
Reviewed-by: Hadriel Kaplan <hadrielk@yahoo.com>