Compatibility with 1.2 scores

* In 1.2 beams cannot span measures. "Middle Beam" markings are ignored in
  this case. In 2.0 the marking is honored.
  Bug: the current implementation does not handle beams spanning pages.
       The resulting layout is undefined.
  (Fixed by replacing "Middle Beam" markings at the beginning of a Measure
   by "Beam Begin" marking)