[pdf] Add support for xmp metadata outside x:xmpmeta element
Based on the [specification](https://wwwimages2.adobe.com/content/dam/acom/en/devnet/xmp/pdfs/XMP SDK Release cc-2016-08/XMPSpecificationPart1.pdf) (Section 7.3.3) the optional x:xmpmeta
element may be placed around the rdf:RDF element.
The element’s namespace URI shall be "adobe:ns:meta/".
Now the functions try to extract the tags from the rdf:RDF
element and if it is NULL
, they try inside the x:xmpmeta
.