NF: add version of Paul Ivanov's slice viewer

@ivanov - many thanks for this - it's very nice, and very useful.

The thing I really miss though, is having cross-hairs showing me the
corresponding current point on all three panels.

I think there's an example here:

http://marsbar.sourceforge.net/_images/roi_view.png

I experimented for a while trying to put the crosshairs on, but it was beyond
my blit fu.  Any pointers as to how to do that?  I mean, put up the
cross-hairs, different on every movement of the slices, but still be able to
blit for speed?