GtkMenuTracker: fix signal handler argument list

The signal handler for the visibility of menu items changing had the
wrong signature, resulting in the GParamSpec from the notify signal
being treated as a boolean (which was always true).  This resulted in
items being added over and over and never being removed.

Fix that...

gtk/gtkmenutracker.c

@@ -219,14 +219,17 @@ gtk_menu_tracker_section_sync_separators (GtkMenuTrackerSection *section,
 
 static void
 gtk_menu_tracker_item_visibility_changed (GtkMenuTrackerItem *item,
-                                          gboolean            is_now_visible,
+                                          GParamSpec         *pspec,
                                           gpointer            user_data)
 {
   GtkMenuTracker *tracker = user_data;
   GtkMenuTrackerSection *section;
+  gboolean is_now_visible;
   gboolean was_visible;
   gint offset = 0;
 
+  is_now_visible = gtk_menu_tracker_item_get_is_visible (item);
+
   /* remember: the item is our model */
   section = gtk_menu_tracker_section_find_model (tracker->toplevel, item, &offset);