[vtkusers] vtk actor inheritance

[John Biddiscombe]

> For now, you can probably achieve the same result if you derive from
> vtkAssembly and put your actor inside the vtkAssembly?

Just add the ID field to vtkProp and rebuild vtk. Then you get what you
wanted without the hassle of messing about with stuff. Probably its a useful
addition to vtkProp to add a user configurable field anyway and we could
stick it in CVS.

vtkGetMacro(int, UserTag);
vtkSetMacro(int, UserTag);

Happy?

JB