Support for menu separators in the GTK tool-bar.

Support for menu separators in the GTK tool-bar.

* src/gtkutil.c (XG_BIN_CHILD): New macro.
(xg_get_menu_item_label, xg_update_menubar)
(xg_update_menu_item, xg_tool_bar_menu_proxy)
(xg_show_toolbar_item, update_frame_tool_bar): Use it.
(separator_names, xg_separator_p): Move to keyboard.c.
(create_menus, xg_update_submenu, update_frame_tool_bar): Use
menu_separator_name_p.

* src/keyboard.c (parse_tool_bar_item): Allow menu separators in
tool-bar maps.
(menu_separator_name_p): New function, from gtkutil.c.
(separator_names): Move from gtkutil.c.

* src/keyboard.h (menu_separator_name_p): Add prototype.

* src/nsmenu.m (name_is_separator): Function deleted.
(addItemWithWidgetValue): Use menu_separator_name_p.

* src/w32menu.c (name_is_separator): Function deleted.
(add_menu_item): Use menu_separator_name_p.