Key names of the document metadata

[matteofg]

This discussion stems from #27. Assuming that \DocumentMetadata will be the sole interface for specifying document metadata, I'd like to start discussing the names of the keys.

Below is a summary table of some keys available through hyperref

hyperref option	PDF property	XMP field
pdftitle	Title	dc:title
pdfauthor	Author	dc:creator
pdfsubject	Subject	dc:description
pdfkeywords	Keywords	pdf:Keywords

I'll leave it to those with more experience in this area to work out the most suitable names. My aim is simply to start a discussion, so that the LaTeX Team can implement the keys that users can access directly through \DocumentMetadata as soon as possible.