struct psppire_output_view

[John Darrington]

I think struct psppire_output_view is needlessly complicated.
It contains a GtkLayout which only ever has one child, namely a single 
GtkDrawingArea.  So far as I can tell, this drawing area is destoyed and 
recreated on every expose event, which has to be both expensive and cumbersome.

It also contains a member called toplevel which is set but never used.

Above all, I have great trouble understanding how it works.  I guess this is 
because it is something created over ten years ago (by myself!) and has evolved 
over time, with a lot of contribution by Ben.

We could try to clean it up, but I wonder if it might not be better to start 
again from scratch?  If we ever want to make pivot tables interactive, this will
probably be essential anyway.

J'