Use Gtk+ tooltips by default for Gtk+ Emacs.

Use Gtk+ tooltips by default for Gtk+ Emacs.

* lisp/cus-start.el (x-gtk-use-system-tooltips): New variable.

* src/gtkutil.c (hierarchy_ch_cb, qttip_cb, xg_prepare_tooltip)
(xg_show_tooltip, xg_hide_tooltip, xg_free_frame_widgets): New
functions.
(xg_create_frame_widgets): Set ttip_* to 0.  Set a dummy tooltip
text so qttip_cb is called.  Connect query-tooltip to qttip_cb.
Remove code that is commented out.

* src/gtkutil.h (xg_free_frame_widgets, xg_prepare_tooltip)
(xg_show_tooltip, xg_hide_tooltip): Declare.

* src/xfns.c (x_gtk_use_system_tooltips): New variable.
(Fx_show_tip): If USE_GTK and x_gtk_use_system_tooltips, call
new gtkutil tooltip functions to show the tooltip.
(Fx_hide_tip): Call xg_hide_tooltip.
(syms_of_xfns): Defvar x-gtk-use-system-tooltips.

* src/xterm.c (x_clear_frame): Check FRAME_GTK_WIDGET (f) before
calling gtk_widget_queue_draw.
(x_free_frame_resources): Call xg_free_frame_widgets.

* src/xterm.h (struct x_output): Add ttip_widget, ttip_window and
ttip_lbl.