Yet another pane pain: allow deleting children using forall_vfunc

Yet another pane pain: allow deleting children using forall_vfunc

We not only need to make sure the iterator remains valid, but also
the object pointed to.

Valgrind trace:
Invalid read of size 8
 Gtkmm2ext::Pane::forall_vfunc(int, void (*)(_GtkWidget*, void*), void*) (pane.cc:617)
 Gtk::Container_Class::forall_vfunc_callback(_GtkContainer*, int, void (*)(_GtkWidget*, void*), void*)
 gtk_container_destroy (gtkcontainer.c:1073)
 g_closure_invoke (gclosure.c:804)
...
 g_object_run_dispose (gobject.c:1084)