Unimplemented graphics properties

[Rik]

Octave does not yet implement all of the graphics properties supported by
Matlab.  For example, in graphics.in.h there is the following note:

    // FIXME: Properties that still don't have callbacks are:
    // monitorpositions, pointerlocation, pointerwindow.
    // Note that these properties are not yet used by Octave, so setting
    // them will have no effect.

Currently, when read, these properties return all zeros.  Would it not be
better to return NA (Not Applicable) to indicate that these are
unsupported?  I think that would be more instructive.

It's a trivial change to the code:

array_property monitorpositions r , Matrix (1, 4, 0.0)
=>
array_property monitorpositions r , Matrix (1, 4, octave_NA)

--Rik