[vtkusers] Get a list of selected graph vertices

[David Doria]

> There are restrictions documented for the vtkHardwareSelector, I am not sure
> graph vertices are candidates for selection.
>
> Hope that helps,
> Jim

I am not sure using the Selector directly is the best way to go - that
was just where my search through the code led me. I see now that the
Selector is a member of the Renderer, that takes away one level of my
mess :)

One of the objects already knows about the selection, because the
default behavior is to highlight the selected vertices/edges! All I
want to know is the IDs of these things that have been selected. The
selection shouldn't have to be performed again.

David