MusicXML: Produce only one output file; optionally use stdin/stdout

MusicXML: Produce only one output file; optionally use stdin/stdout

-) So far, I generated a file.ly for the score and file-defs.ly for the
   definitions. Actually, I don't see why this should be neccessary / useful
   as it only clutters the outdir, breaks the links in the examples, makes
   the use of stdout (for piping MusicXML through) impossible etc.
   So, from now on, only one file (file.ly) will be generated, which contains
   all lilypond code from the conversion.
-) Also implemented reading from stdin (if filename is -) and writing to
   stdout (if --out=-).