Show an entry on the system tray indicator when a portal is needed

Summary: This way it's still accessible if I lose the notification somehow.

Test Plan: I don't get it when connected normally on the internet. I managed to reach the portal when on the plane.

Reviewers: #plasma, jgrulich

Reviewed By: jgrulich

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22079