tooltip: set window type hint for self-drawn tooltips to GDK_WINDOW_TYPE_HINT_TOOLTIP

libcanberra generates specific tooltip popup sounds and for that
recognizes the tooltip windows by the GtkWindowTypeHint set for them.

This trivial patch simply sets the hint for the self-drawn tooltips
metacity uses.

https://bugzilla.gnome.org/show_bug.cgi?id=609585

src/ui/fixedtip.c

@@ -71,6 +71,8 @@ meta_fixed_tip_show (Display *xdisplay, int screen_number,
   if (tip == NULL)
     {      
       tip = gtk_window_new (GTK_WINDOW_POPUP);
+      gtk_window_set_type_hint (GTK_WINDOW(tip), GDK_WINDOW_TYPE_HINT_TOOLTIP);
+
       {
         GdkScreen *gdk_screen;
 	GdkRectangle monitor;