Moved ViewerBase class out into its own header and source file.

Improved the support for stats collection in CompositeViewer/StatsHandler.

src/osgViewer/Viewer.cpp

@@ -1408,7 +1408,6 @@ void Viewer::eventTraversal()
         getStats()->setAttribute(_frameStamp->getFrameNumber(), "Event traversal time taken", endEventTraversal-beginEventTraversal);
     }
 
-
 }
 
 void Viewer::updateTraversal()
@@ -1585,6 +1584,11 @@ void Viewer::getScenes(Scenes& scenes, bool onlyValid)
     scenes.push_back(_scene.get());
 }
 
+void Viewer::getViews(Views& views, bool onlyValid)
+{
+    views.push_back(this);
+}
+
 struct LessGraphicsContext
 {
     bool operator () (const osg::GraphicsContext* lhs, const osg::GraphicsContext* rhs) const