nextplot and replacechildren value

[Ben Abbott]

Both the figure and axes objects have "nextplot" properties.

Matlab's implementation permits the following values, with the defaults 
enclosed by {}.

>> set(gca,'nextPlot')
[ new | add | {replace} | replacechildren ]

>> set(gcf,'nextPlot')
[ new | {add} | replace | replacechildren ]

For axes, Octave's implementation is currently (from graphics.cc.in)

      radio_property nextplot , "add|replace_children|{replace}"

... and for figures

      radio_property nextplot , "new|{add}|replace_children|replace"

I'm presently building and will test with the attached changeset.

Any concerns about pushing a changeset with these changes?

Ben

changeset.patch
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