Stub for ly to xml export using engravers

[John Mandereau]

After some discussion on developing a ly to xml export program, I
thought that the using engravers to plug such a converter to ly input
at a processing stage that would avoid parsing ly files and allow to
get any ly file converted, provided that (from advice from Mike) you
take into account how the iterators play in this game.

So, here's a stub for possibly starting a ly2xml converter with this approach.

Best,
John

ly2xml-engravers.ly
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