This patch implements the "proximity delay" when displaying tooltips.
The rationale for this patch has been posted in my message to gtk-list
with Message-ID <87zp6h37bs.fsf@pc-hrvoje.srce.hr>.  Here is a
summary:

    Gtk lets the application specify the delay parameter of a tooltip
    object.  The delay is the time the mouse needs to hover over the
    object for the tooltip to appear, the default delay being half a
    second.

    However, when a tooltip object encompasses several close widgets (e.g.
    a toolbar), and you want to see the tooltip of each button, you have
    to wait for the delay period to pass over every widget, which tends to
    get annoying.  Many other windowing systems (e.g. Windows) show the
    following tooltip immediately.

This patch implements that behaviour by introducing the notion of
"proximity delay".  If less than 200 msecs pass after a tooltip has
been destroyed, the next tooltip within the same tooltip object will
be shown immediately.  The proximity delay can be changed using the
new function gtk_tooltips_set_proximity_delay().

hangeLog entry (also present in the patch) is as follows:

1999-02-24  Hrvoje Niksic  <hniksic@srce.hr>

        * gtk/gtktooltips.c (gtk_tooltips_set_proximity_delay): New
        function.
        (gtk_tooltips_proximity_timeout): Ditto.
        (gtk_tooltips_set_active_widget): Setup the timeout when hiding
        tip_window.
        (gtk_tooltips_event_handler): If a tooltip was recently destroyed,
        draw the new one immediately.

        * gtk/gtktooltips.h (struct _GtkTooltips): New members
        proximity_delay and proximity_timer_tag.