(Optionally) embed ly source files inside generated PDF

(Optionally) embed ly source files inside generated PDF

By using the new -dembed-source-code command line switch,
users may now include their LilyPond source code in the
generated PDF (using the pdfmark EMBED feature); although
the PDF is not modified in any visible way, embedded .ly
files should appear in viewers that support this feature
(other viewers should degrade gracefully).

This patch adds a new ly:source-files function (with
an optional parser arg) that allows to retrieve the list
of (non distribution-provided) ly files involved in
making the current score.