Adding NAT rule based on a NAT rule moves it to bottom of list
When adding a new NAT rule based on another NAT rule, it adds the new one to the bottom of the list. Doesn't matter much, but it should have the same behavior as on firewall rules where it adds it beneath the rule it was based on.
Add NAT rule based on NAT rule below it instead of at the bottom of the list. Fixes #1118