Don't use deprecated functions when compiling with Gtk3.

Don't use deprecated functions when compiling with Gtk3.

* gtkutil.c (gtk_scrollbar_new, gtk_box_new): Define when HAVE_GTK3 is
not defined (Bug#11768).
(xg_create_frame_widgets): Use gtk_plug_new_for_display (Bug#11768).
(xg_create_frame_widgets, create_dialog, xg_get_file_with_chooser)
(make_widget_for_menu_item, xg_make_tool_item): Use gtk_box_new
followed by gtk_box_set_homogeneous (Bug#11768).
(xg_update_menu_item): Use GTK_IS_BOX (Bug#11768).
(update_theme_scrollbar_width, xg_create_scroll_bar): Use
gtk_scrollbar_new (Bug#11768).
(xg_event_is_for_scrollbar): Use Gdk Device functions for HAVE_GTK3.
(is_box_type): New function (Bug#11768).
(xg_tool_item_stale_p): Call is_box_type.
(xg_initialize): Get settings by calling gtk_settings_get_for_screen
with default display (Bug#11768).