Follow-up to "Do not depend on `Viewer.h`"

That commit was not complete:
> Author: Laurent Rineau <laurent.rineau@cgal.org>
> Date:   Fri Apr 1 16:11:52 2016 +0200
>
>     Do not depend on `Viewer.h`
>
>     Only `Viewer_interface.h` is needed.

Once `Viewer.h` is removed from a few `.h` file, then
`Viewer_interface.h` must be included in `.cpp` files that use the
`Viewer_interface` API.

Polyhedron/demo/Polyhedron/Plugins/PCA/Scene_polyhedron_transform_item.cpp

@@ -1,6 +1,7 @@
 #include "Scene_polyhedron_transform_item.h"
 #include "Kernel_type.h"
 #include "Polyhedron_type.h"
+#include <CGAL/Three/Viewer_interface.h>
 
 Scene_polyhedron_transform_item::Scene_polyhedron_transform_item(const qglviewer::Vec& pos,const Scene_polyhedron_item* poly_item_,const CGAL::Three::Scene_interface*):
     Scene_item(NbOfVbos,NbOfVaos),

Polyhedron/demo/Polyhedron/Plugins/Surface_mesh_deformation/Scene_edit_polyhedron_item.cpp

@@ -3,6 +3,7 @@
 #include "opengl_tools.h"
 #include "create_sphere.h"
 #include "Scene_edit_polyhedron_item.h"
+#include <CGAL/Three/Viewer_interface.h>
 #include <boost/foreach.hpp>
 #include <algorithm>
 #include <QTime>

Polyhedron/demo/Polyhedron/Scene_nef_polyhedron_item.cpp

@@ -1,4 +1,5 @@
 #include "Scene_nef_polyhedron_item.h"
+#include <CGAL/Three/Viewer_interface.h>
 #include "Scene_polyhedron_item.h"
 #include "Nef_type.h"
 #include "Polyhedron_type.h"

Polyhedron/demo/Polyhedron/Scene_polyhedron_selection_item.h

@@ -14,6 +14,11 @@
 #include <CGAL/Polygon_mesh_processing/connected_components.h>
 #include "Polyhedron_demo_detect_sharp_edges.h"
 
+// Laurent Rineau, 2016/04/07: that header should not be included here, but
+// only in the .cpp file. But that header file does contain the body of a
+// few member functions.
+#include <CGAL/Three/Viewer_interface.h>
+
 #include <fstream>
 #include <boost/foreach.hpp>
 #include <boost/unordered_set.hpp>