Added DragCallback class to help dialog dragging support.
Introduced a new Widget::computeExtentsPositionInLocalCoordinates() method that intersects with a ray through mouse pointer and the extents of the widget.


git-svn-id: http://svn.openscenegraph.org/osg/OpenSceneGraph/trunk@14429 16af8721-9629-0410-8352-f15c8da7e697