Basic musicXML export

Some kind of start for a musicXML export. Here is some initial thoughts:

As I first read here https://github.com/openlilylib/ly2xml/wiki/Requirements-and-development-ideas, Frescobaldi's internal representation of the source code could be used to convert the ly-file to a musicXML-file. To learn more about Frescobaldi in general and the internal representation in particular I started to explore this possibility. 

As there currently is no known development of a ly2musicxml function I think an internal solution in Frescobaldi could be constructive. Perhaps could it also be contributive to the ly2musicxml itself. Except from the possible complexity of a lilypond input there is also the problem with the things that are not present in the source or at least not mandatory; such as beaming, or a major concern from my part from the beginning - barlines. About the barlines I read in the TODO document about auto-inserted barchecks. Maybe the effort in that case should be directed towards this function.

I think a realistic first goal for the export would be to handle a file imported by musicxml2ly. In that way Frescobaldi wouldn't be a dead-end; you could import a musicXML-file, make some changes and export it again. I don't know if anyone in the LilyPond community would find this useful. Perhaps it could be a first step towards LilyPond from for example working with Sibelius. Personally I will use it to create audio simulations. Musicians often request an audio simulation of a score and the output from Finale is so much better than a simple midi-file.