Only run one toplevel callback per event loop iteration.

This change attempts to reinstate as a universal property something
which was sporadically true of the ad-hockery that came before
toplevel callbacks: that if there's a _very long_ queue of things to
be done through the callback mechanism, the doing of them will be
interleaved with re-checks of other event sources, which might (e.g.)
cause a flag to be set which makes the next callback decide not to do
anything after all.

[originally from svn r10040]

unix/gtkwin.c

@@ -1422,7 +1422,8 @@ static gint idle_toplevel_callback_func(gpointer data)
         run_toplevel_callbacks();
     }
 
-    gtk_idle_remove(inst->toplevel_callback_idle_id);
+    if (!toplevel_callback_pending())
+        gtk_idle_remove(inst->toplevel_callback_idle_id);
 
     return TRUE;
 }