Export "first" beam.
This works only with regular eighth note beams. Anything else won't work.
Only beam nr 1 is created and ended.
But if more than one beam is involved they have to be encoded all.

That means
a) we have to check the number of beams at [ and ]
b) we have to check at each note if it is beamed, and if so
   the current number of beams has to be calculated.
(seems challenging)

As described in
http://lists.gnu.org/archive/html/lilypond-user/2018-05/msg00128.html
there is the possibility to simply encode *one* beam and add
<beam>continue</beam> to each enclosed note.

This is no correct MusicXML, but it will correctly be loaded by MuseScore
and Verovio (test at http://www.verovio.org/musicxml.html).

If we agree to stick with that halfway solution (at least for now) I think we
can now (after adding a test file) merge this.