This change provides developers with a way to override the
`vtkExtractSelection` instance used internally by `vtkConvertSelection`.

Also, when processing `QUERY` selections, try to use the externally
provided extract-selection filter when converting to index selections.

This change allows ParaView (and other applications) to support
`vtkSelectionNode`s of type `QUERY`, which VTK does not currently
handle.

Change-Id: I4a94e393796ae6ea0905895f60a2f62b2c78315d