System Monitor Library

Right now, we have scattered code for reading various system stats:

• https://gitlab.xfce.org/apps/xfce4-taskmanager/
• https://gitlab.xfce.org/panel-plugins/xfce4-systemload-plugin/
• https://gitlab.xfce.org/panel-plugins/xfce4-diskperf-plugin/
• https://gitlab.xfce.org/panel-plugins/xfce4-cpufreq-plugin/
• https://gitlab.xfce.org/panel-plugins/xfce4-cpugraph-plugin/
• https://gitlab.xfce.org/panel-plugins/xfce4-wavelan-plugin/
• https://gitlab.xfce.org/panel-plugins/xfce4-sensors-plugin/

It would be great if we could consolidate their backends into a single xfce_system_stats library here to reduce code duplication.