Revise interface of port forwarder

This eases the usage of port forwarder as the caller code doesn't need
to store previous used port and now can rely on port forwarder doing
all the hard work.

PR #17967.

src/webui/webui.h

@@ -66,5 +66,4 @@ private:
     QPointer<Http::Server> m_httpServer;
     QPointer<Net::DNSUpdater> m_dnsUpdater;
     QPointer<WebApplication> m_webapp;
-    quint16 m_port = 0;
 };