application: Ignore windows that are not NautilusWindow when using gtk_application_get_windows () to avoid crashes when accessing things like GtkDialog.

Nautilus crashes on startup if the "required directories missing" dialog
appears. This is because the code is mistakenly assuming that
gtk_application_get_windows() only contains objects of type NautilusWindow. The
dialog is an object of type GtkDialog.

https://bugzilla.gnome.org/show_bug.cgi?id=728765