MusicXML: Support for upbeats and partial/implicit measures

MusicXML: Support for upbeats and partial/implicit measures

-) In the Duration class, don't write out e.g. *3/1 for the factor.
   In this case, *3 is much better
-) Add support for implicit measures, i.e. measures in MusicXML that do
   not count. These are typically used for upbeats and for places where
   a repeat or clef/key change occurs inside a measure. In this case,
   the measure is split into to <measure> elements, the second one with
   the implicit attribute set to "yes". Now we don't start a measure
   in lilypond for those implicit measures.
-) Implicit measures at the very beginning are now converted as \partial X
-) Don't print out | %0 at the very beginning (i.e. for the measure with
   number 0)

Signed-off-by: Reinhold Kainhofer <reinhold@kainhofer.com>