Introduced Widget::WidgetStateSet to help localize the StateSet set up by Widget implementations from being serialized or
affecting what end users apply via the standard Node::s/getStateSet().

Further work on TabWidget.


git-svn-id: http://svn.openscenegraph.org/osg/OpenSceneGraph/trunk@14440 16af8721-9629-0410-8352-f15c8da7e697