Callbacks for data objects

[John Darrington]

This has been at the back of my mind for some time now, but we've
reached the stage where is has to be addressed, if the GUI is going to
advance any further.

Several of the structures in src/data are going to need callbacks.
This is because the GUI needs to dynamically update its display as and
when they change.  So far, I've worked around this issue, by making
wrappers in the GUI, which issues the callbacks.  However, if the GUI
calls a procedure (such as RANK), then the dictionary can change in a
way that the GUI can't possibly know about.

I don't think this is a big issue for dictionary.c, since it's quite
well encapsulated.    We simply pass in a function pointer at time of
construction and the mutators invoke this function.  However,
variable.c is another story because it's currently so transparent.  I
think we will need to make struct variable an opaque type, provide
accessor mutator functions, and update dependent code accordingly.
Other things which may need callbacks are casefile.c and possibly
struct dataset (from procedure.c)

It's another change which is quite pervasive, but I can't see any way
around it, if the GUI's going to work.  Any comments?

J'



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