Updated changelog
UI: Separated checkbox for IPv6 enablement.

All references to old forwarding flag were renamed to forward_v4. E.g.
RoutingClass::Forward -> RoutingClass::Forward_v4 and similarly in other
cases.

Also some cleanup work is present. Mainly removing blank lines.
Added backend for separated IPv6 enablement and some cleanup.

Cleanup is related to YCP -> Ruby conversion.
Minor update in old testsuite to keep things working.