Use non-deprecated Gtk+ functions, add changes for Gtk+3 with GSEAL_ENABLE.

Use non-deprecated Gtk+ functions, add changes for Gtk+3 with GSEAL_ENABLE.

* configure.in: Add --with-x-toolkit=gtk3. Remove HAVE_GTK_MULTIDISPLAY,
check for gtk_file_chooser_dialog_new, and HAVE_GTK_FILE_BOTH (implied
by minimum required Gtk+ 2.6).  Add checks for functions introduced
in Gtk+ 2.14 or newer.

* xterm.h (gtk_widget_get_window, gtk_widget_get_mapped)
(gtk_adjustment_get_page_size, gtk_adjustment_get_upper): New
defines based on what configure finds.

* xterm.c (XTflash): Use gtk_widget_get_window.
(xg_scroll_callback): Use gtk_adjustment_get_upper and
gtk_adjustment_get_page_size.
(handle_one_xevent): Use gtk_widget_get_mapped.
(x_term_init): Remove HAVE_GTK_MULTIDISPLAY and associated error
messages.

* xmenu.c (create_and_show_popup_menu): Call gtk_widget_get_mapped.

* gtkutil.h: Replace HAVE_GTK_FILE_BOTH with
HAVE_GTK_FILE_SELECTION_NEW.

* gtkutil.c (xg_display_open, xg_display_close): Remove
HAVE_GTK_MULTIDISPLAY, it is always defined.
(xg_display_open): Return type is void.
(gtk_widget_set_has_window)
(gtk_dialog_get_action_area, gtk_dialog_get_content_area)
(gtk_widget_get_sensitive, gtk_adjustment_set_page_size)
(gtk_adjustment_set_page_increment)
(gtk_adjustment_get_step_increment): #define these if not found
by configure.
(remove_submenu): New define based on Gtk+ version.
(xg_set_cursor, xg_frame_resized, xg_event_is_for_scrollbar): Use
gtk_widget_get_window.
(xg_frame_resized, xg_update_frame_menubar): Use gtk_widget_get_mapped.
(xg_create_frame_widgets): Use gtk_widget_set_has_window.
(create_dialog): Use gtk_dialog_get_action_area and
gtk_dialog_get_content_area.
(xg_uses_old_file_dialog, xg_get_file_name): Remove HAVE_GTK_FILE_BOTH
and HAVE_GTK_FILE_CHOOSER_DIALOG_NEW.  File chooser is always
available, so checking for HAVE_GTK_FILE_SELECTION_NEW is enough.
(xg_update_menubar, xg_update_submenu, xg_show_toolbar_item): Use
g_object_ref and g_object_unref.
(xg_update_menu_item, xg_tool_bar_menu_proxy): Use
gtk_widget_get_sensitive.
(xg_update_submenu): Use remove_submenu.
(xg_update_scrollbar_pos): Don't use GtkFixedChild, use child
properties instead to get old x and y position.
(xg_set_toolkit_scroll_bar_thumb): Use gtk_adjustment_get_page_size,
gtk_adjustment_get_step_increment, gtk_adjustment_set_page_size,
gtk_adjustment_set_step_increment and gtk_adjustment_set_page_increment.
(xg_get_tool_bar_widgets): New function.
(xg_tool_bar_menu_proxy, xg_show_toolbar_item)
(update_frame_tool_bar): Call xg_get_tool_bar_widgets.
(toolbar_set_orientation): New #define based on if configure
finds gtk_orientable_set_orientation.
(xg_create_tool_bar): Call toolbar_set_orientation.
(xg_make_tool_item, xg_show_toolbar_item): Call gtk_box_pack_start
instead of gtk_box_pack_start_defaults.