Part-of: <https://gitlab.gnome.org/GNOME/gnome-control-center/-/merge_requests/2000>
Per mockups in https://gitlab.gnome.org/Teams/Design/settings-mockups/-/raw/master/system/system-panel.png This adds a thin abstract CcSystemPage object with a "summary" property. Each CcListRow::subtitle is binded to its corresponded CcSystemPage::summary. See #2241
See #2241