Entrust Gtk with handling tooltips

Entrust Gtk with handling tooltips

Previously, tooltips for menu items are below the menu.
To solve it, set tooltip text on menu items in advance, and entrust
gtk with handling tooltips.

* src/gtkutil.c (xg_show_tooltip): New function for pgtk to set
tooltip text on the widget.
(xg_hide_tooltip): New function for pgtk to clear tooltip text on the
widget.
(xg_create_frame_widgets): Don't set initial tooltips text, and don't
use qttip_cb.
(xg_create_frame_outer_widgets): Ditto.
(make_menu_item): Set tooltip text on the menu item.
* src/gtkutil.h: New declaration of xg_show_tooltip for pgtk.
* src/pgtkfns.c (Fx_show_tip): Calls xg_show_tooltip, and always ok.