#370 set key horizontal and invert

[Bernhard]

When setting the key property

set key invert

The ordering of the labels in the legend will invert.

If I want to invert the order in a horizontal key, I would naturally

set key horizontal invert

However, this is not supported. This would be a nice feature.

Related request: https://sourceforge.net/p/gnuplot/feature-requests/339/

[Masahiro Takahashi]

I also want this feature.