Add an indicator for when there is something access the display server
remotely. This could be 1) remote desktop, 2) screen cast or 3) remote
control, but all effectively applications using
org.freedesktop.portal.ScreenCast or org.gnome.portal.RemoteDesktop as
well as gnome-remote-desktop using the corresponding org.gnome.Mutter
APIs directly.
As it is now, it'll simply show a single icon for when anything is
having an active session, and a single action "Turn off" that'll close
every active session.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/160
