system: Add title to system panel
requested to merge gautham_x/gnome-control-center:gsoc23-sytem-window-title into gsoc23/new-system-panel
Adding "System" as title to the "System" panel, per mockups in https://gitlab.gnome.org/Teams/Design/settings-mockups/-/blob/master/system/system-panel.png
See #2241 (closed)