backend: Add support for xmpRights:UsageTerms in PDF

The Creative Commons instructions about how to get the license
information it's ambiguous about the human readable statement
about the license. The current method for checking this
information is using xmpRights:UsageTerms, and Evince only
checks dc:rights, the previous recommended method (tough it
is present on the template and applications recommended by CC) .

Fixes #128

Signed-off-by: Germán Poo-Caamaño <gpoo@gnome.org>
4 jobs for wip/gpoo/128-xmpRights in 5 minutes and 57 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #778864
flatpak
flatpak

00:05:57

manual #778861
allowed to fail manual
master
passed #778862
meson

00:00:41

manual #778863
allowed to fail manual
meson-gtk-doc