Adds Date Time panel to the system panel as a page. https://gitlab.gnome.org/Teams/Design/settings-mockups/-/blob/master/system/system-panel.png See #2241
Both Cities and Regions need to be translated. Also make sure that we sort them properly, and use symbolic names for COLUMNS in the region liststore. https://bugzilla.gnome.org/show_bug.cgi?id=630264