Bug#873951: xpdf: new major upstream version 4.x

[Florian Schlichting]

xpdf 4.00 switched from motif to QT. IMHO this eliminates the only
reason to use xpdf instead of evince, given that both are based on
poppler in Debian. I think we should keep maintaining 3.04 as a fork as
long as this is feasible, although anyone is of course welcome to
backport fixes and features.