Notebook_Data: Destructor: Work around crash during shutdown.

* glom/utility_widgets/notebook_noframe.[h|cc]:
  Add a hacky remove_all_pages_without_signalling() method
  to avoid us calling get_parent() while this widget is being
  removed, because that is causing get_parent() to be called on
  a GtkWidget that is no longer valid. This has started happening
  recently so I suspect a change in GTK+ or gtkmm.
  I have not put in the time to identify what is really going
  wrong.
* glom/mode_data/notebook_data.cc: Destructor: Call the new
  workaround method.