Draft: temp/gnome-clocks: suspend via timerfd

This MR isn't here to get merged. It will rather serve as an tracking issue for current problems.

• systemd integration doesn't work (child user-namespace can't call timerfd_create, although getpcaps,pscaps report okay)
  • currently work-arounded with setcap, but that's no solution....
• when no service is running, dbus activation won't work