Check for deprecated gtk tearoff menu item in configure.

Check for deprecated gtk tearoff menu item in configure.

* configure.ac: Check for GtkHandlebox.
Check for GtkTearoffMenuItem.

* src/gtkutil.c (TOOLBAR_TOP_WIDGET): New macro.
(xg_pack_tool_bar): Use TOOLBAR_TOP_WIDGET, condition out use of
handlebox_widget.  Set toolbar_in_hbox to false/true, set
toolbar_is_packed to true.
(xg_update_tool_bar_sizes): Use widget returned by TOOLBAR_TOP_WIDGET.
(update_frame_tool_bar): Check toolbar_is_packed for packing.
Show all on TOOLBAR_TOP_WIDGET.
(free_frame_tool_bar): Check toolbar_is_packed.  Use widget returned
by TOOLBAR_TOP_WIDGET.
(xg_change_toolbar_position): Use widget returned by TOOLBAR_TOP_WIDGET.
Check toolbar_is_packed.
(xg_have_tear_offs, tearoff_remove, tearoff_activate): Condition on
HAVE_GTK_TEAROFF_MENU_ITEM_NEW.
(xg_have_tear_offs): When ! HAVE_GTK_TEAROFF_MENU_ITEM_NEW, return
false.
(create_menus): Create tearoff only if HAVE_GTK_TEAROFF_MENU_ITEM_NEW.
(xg_update_menubar): Update title only if
HAVE_GTK_TEAROFF_MENU_ITEM_NEW.
(xg_update_submenu): Skip tearoff only if
HAVE_GTK_TEAROFF_MENU_ITEM_NEW.
(xg_initialize): Initialize xg_detached_menus only if
HAVE_GTK_TEAROFF_MENU_ITEM_NEW.