networking: Emit "changed" event when readonlyness changes

We can't rely on the GetZones call to emit a "changed" event.  It
might run into an error, such as when the firewall is off.

Closes cockpit-project#13703

Author: mvollmer

pkg/networkmanager/firewall-client.js

@@ -299,6 +299,7 @@ initFirewalldDbus();
 cockpit.spawn(['sh', '-c', 'pkcheck --action-id org.fedoraproject.FirewallD1.all --process $$ --allow-user-interaction 2>&1'], { superuser: "try" })
         .done(() => {
             firewall.readonly = false;
+            firewall.debouncedEvent('changed');
             firewall.debouncedGetZones();
         });