Add remote access indication and control

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.

---

This is my interpretation of https://wiki.gnome.org/Design/OS/SystemStatus .

Here are screenshots for how it looks with the current implementation: