doc: Create new Graphic Properties Index (bug #52812). * doc/interpreter/pr-idx.txi: New index file. * doc/interpreter/module.mk: Add pr-idx.txi to build system. * genpropdoc.m: Place graphic properties in to @prindex. * octave.texi: Use "@defindex pr" to create a new index. Add "Graphics Properties Index" to menu. "@include pr-idx.texi" as last index after Operators Index. * plot.txi: Put "ObjectXXX Properties" into prindex and use @sortas to have overall list appear first in index.

gnulib = http://hg.octave.org/gnulib