Fix 'local' icons

Apparently, in GNOME Shell 3.32, both GtkIconTheme's append_search_path() and add_resource_path() no longer work as expected with St's Icon, with the result that we cannot simply append a path to the icon theme and refer to icons by just their names (using St.Icon's 'icon_name' field), like we currently do.
So, as a work-around, move to Gio's Icon's (GIcon) and use them in St's Icon's ('gicon' field).