Include ly.music (from Frescobaldi) in LilyPond distribution

[Urs Liska]

I suggest that we include Frescobaldi's new ly.music library into LilyPond.
https://github.com/wbsoft/frescobaldi/wiki/Notes-about-ly.music
It is a library _and_ a script to process .ly files.

For example it is able to construct a DOM for a LilyPond score and 
generate the .ly file from it. Apart from any other uses one might come 
up with this would make it easier to create the resulting LilyPond files 
from midi2ly or musicxml2ly

I see the following tasks:
- find a suitable place for the library in the source and build 
(probably below out/lib)
- make it accessible (export the path)
- incorporate it in the build system
- create a straightforward way to pull updates into LilyPond's source dir.

What do you think?

Urs