system-monitor: Add new extension

A long time ago, we used to include a system monitor extension, that added CPU/memory graphs to the (long gone) message tray.

However demand for this type of extensions hasn't died down, to the point where RHEL includes a revived version of the old extension.

Account for that demand by adding a newly written system-monitor extension that has been properly designed, and hopefully does not bring back the CPU/memory issues of the previous one (unlikely without the graphs) …