Support firewalld zones

(I am filing this here because there does not seem to be a component at https://bugs.kde.org/ for this project yet.)

Firewalld has a central concept called "zones". Those are the main setting to make in the firewalld. They correspond to a whole profile of rules. All rules are part of one or more zones. Switching the zone switches the entire rule set. (Those zones are implemented deep in the backend, not in the UI, i.e., the UI should use the backend's zones and not attempt to come up with its own.) Think of it as the equivalent of sysv runlevels or systemd targets, but for a firewall.

Unfortunately, this concept does not seem to be implemented in Plasma Firewall yet.