Making Octaviz compatible with handle graphics

[Jordi Gutiérrez Hermoso]

Last night I got myself to hacking Octaviz to fix some longstanding
things about it that have been nagging me and thought of others things
I could add to it. VTK does seem to make quite a few features easy to
do, once one manages to navigate its horrendous documentation.

Before I actually commit any more time to this, I thought I might as
well make it so that it's compatible with Octave's current
implementation of handle graphics, which was added after Octaviz's
release (and Octaviz development seems stalled right now, no release
in almost a year).

Er. So how should I go about doing this?

- Jordi G. H.