Avoid a segfault in `load_plugin`

Before that patch `accepted_keywords` was a dangling reference to an
object that was destroyed at the end of the constructor of the class
`Polyhedron_demo`.

Now, that `QStringList` is copied, but it is a short list of short
strings.

Polyhedron/demo/Polyhedron/MainWindow.h

@@ -454,7 +454,7 @@ private:
   bool bbox_need_update;
   QMap<QString, QPair<QStringList, QString> >plugin_metadata_map;
   QMap<QString, bool> ignored_map;
-  const QStringList& accepted_keywords;
+  QStringList accepted_keywords;
 
 private:
   QMap<QAction*, QMenu*> action_menu_map;