[PATCH] Postscript extension : comment before and after page

see http://wiki.apache.org/xmlgraphics-fop/ExtensionsForPostScript for full
details of <ps:ps-comment-before/> and <ps:ps-comment-after/>.

The requirement for the comment-before and comment-after is for post-processing
the postscript output. Both these elements may only be declared as child nodes
of fo:declarations and/or fo:simple-page-master.

In a fo:declarations context, ps-comment-before is placed in the postscript
output just before the %%EndComments DSC comment in the header and
ps-comment-after is placed just after the %%Trailer DSC comment in the footer of
the output.

In a fo:simple-page-master context, ps-comment-before is placed in the
postscript output just after the %%BeginPageSetup DSC comment and a
ps-comment-after just after the %%PageTrailer DSC comment in each page of a
simple page master referring page sequence.

Patch (with dependency on
http://issues.apache.org/bugzilla/show_bug.cgi?id=42144 being applied) to follow.