Partially fixes https://gitlab.gnome.org/GNOME/gnome-control-center/-/issues/2541
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
Adding Region & Language page to the system panel. https://gitlab.gnome.org/Teams/Design/settings-mockups/-/blob/master/system/system-panel.png See #2241