[osg-submissions] Reworked osgviewerWX example

[Paul Melis]

Hello Robert,

Here is a reworked version of the osgviewerWX example. It changes the 
GraphicsWindowWX to only inherit from osgViewer::GraphicsWindow and adds 
a standalone widget, called OSGCanvas, that derives from wxGLCanvas. 
This solves a problem with the GraphicsWindowWX instance being 
destructed twice (see "Crash in osgviewerWX" of June 12th on osg-users). 
At program exit, the main frame deletes all of its children widgets and 
therefore calls GraphicsWindowWX's destructor, bypassing OSG's reference 
counting. The GraphicsWindowWX instance is then later destructed a 
second time when the reference held by osg::Camera goes to zero. This 
bug isn't exposed by the example directly, but if people are going to 
use the example as a basis (like the poster in the mentioned thread) 
they very likely will run into this problem.

Paul