The GtkHSeparator widget is a horizontal separator, used to group the
widgets within a window. It displays a horizontal line with a shadow to
make it appear sunken into the interface.
PLEASE NOTE: The GtkHSeparator widget is not used as a separator within menus.
To create a separator in a menu create an empty GtkSeparatorMenuItem
widget using gtkSeparatorMenuItemNew and add it to the menu with
gtkMenuShellAppend.