remove warnings about hiding an init method

Polyhedron/demo/Polyhedron/Polyhedron_demo_join_polyhedra_plugin.cpp

@@ -21,6 +21,7 @@ class Polyhedron_demo_join_polyhedra_plugin:
   QAction* actionJoinPolyhedra;
 public:
   QList<QAction*> actions() const { return QList<QAction*>() << actionJoinPolyhedra; }
+  using Polyhedron_demo_plugin_helper::init;
   void init(QMainWindow* mainWindow, Scene_interface* scene_interface, Messages_interface* /* m */)
   {
     actionJoinPolyhedra= new QAction(tr("Join selected polyhedra"), mainWindow);

Polyhedron/demo/Polyhedron/Polyhedron_demo_polyhedron_stitching_plugin.cpp

@@ -22,6 +22,7 @@ class Polyhedron_demo_polyhedron_stitching_plugin :
   QAction* actionStitchBorders;
 public:
   QList<QAction*> actions() const { return QList<QAction*>() << actionDetectBorders << actionStitchBorders; }
+  using Polyhedron_demo_plugin_helper::init;
   void init(QMainWindow* mainWindow, Scene_interface* scene_interface, Messages_interface* /* m */)
   {
     actionDetectBorders= new QAction(tr("Detect polyhedron boundaries"), mainWindow);

Polyhedron/demo/Polyhedron/Polyhedron_demo_selection_plugin.cpp

@@ -29,6 +29,7 @@ public:
   }
   void print_message(QString message) { messages->information(message); }
   QList<QAction*> actions() const { return QList<QAction*>() << actionSelection; }
+  using Polyhedron_demo_plugin_helper::init;
   void init(QMainWindow* mainWindow, Scene_interface* scene_interface, Messages_interface* m) {
     mw = mainWindow;
     scene = scene_interface;