revive compare_plot_demos

[Andreas Weber]

Dear maintainers,

due to the many bug reports related to printing plots and regressions in
plotting demos I would like to revive
http://octave.sourceforge.net/compare_plots/

Here is the initial posting from Sören:
http://octave.1599824.n4.nabble.com/Comparing-plots-to-Matlab-td3068200.html

And this is the last proposed patch from Ben I could found (which was,
to my knowledge, never applied):
http://octave.1599824.n4.nabble.com/demo-syntax-and-using-dump-demos-to-compare-to-ML-tp4316479p4319733.html

Because it's difficult to compare the plots automatically I think a cool
feature would be a rating function where users can give points for each
gnuplot, fltk (and qt in the future) and submit it at the end. For
example 10 points is as good as or even better as matlab, 0 points is
absolutely useless or an empty plot.

What do you think?
-- Andy