[poppler] XMP support

[Albert Astals Cid]

We have "very basic" XMP metadata support, i.e. we "can read it" in 
Catalog::readMetadata and pdfinfo -meta dumps it

But that's it.

The ISO spec says that "the metadata stream shall be taken as authoritative 
[given some conditions]" and we're not obeying that in say pdfinfo.

I guess we should at least support the dc: and xmp: entries that are similar 
to the catalog info document, e.g. dc:title dc:creator xmp:CreateDate

Question is where to do this:
 * in pdfinfo
 * in poppler core

In pdfinfo is easier since we don't have to build lots of classes around it, 
but that means that for example I would have to do the same code in Okular 
which is not particularly great.

Opinions?

Cheers,
  Albert