Check for deprecated gtk_handle_box in configure.

Check for deprecated gtk_handle_box in configure.

* configure.ac: Check for GtkHandlebox.

* 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.

* src/xterm.h (struct x_output): Surround handlebox_widget with
#ifdef HAVE_GTK_HANDLE_BOX_NEW.  toolbar_is_packed is new,
toolbar_in_hbox is bool.