Look for opportunities to improve PDF metadata value assignment

[dominicus]

LilyPond has functionality to carry over \header fields into PDF metadata.
For example, \title of the piece by default is carried over as title for the PDF file. Others: subtitle, copyright, author, composer.
It's possible to customize what values are assigned PDF metadata vs. values Lilypond uses to typeset by defining a new header with "pdf" prefix.
The suggestion is to review mandatory \header fields, and look for opportunities to automate more meaningful tags for PDF.
For example:

• By default, we set \copyright to blank. Adding a header \pdfcopyright = "CC 4.0"
• It's common practice to add dates of birth-death to \composer, while these may not be useful as PDF metadata.