backend: Add support for checking 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 (closed)
This is patch submitted by @jjmarin and made a MR, if I understood correctly as I adjusted to the current code. I have to find a document to test it.