MusicXML: Implement metronome mark (tempo marks)

MusicXML: Implement metronome mark (tempo marks)

MusicXML supports multiple types of metronome marks:
-) A simple one: note = beats (=> Lilypond \tempo note = beats)
-) note = other note (=> Lilypond has to use markup, no effect on
   midi :-((
-) A very advanced and possibly complex one, which uses <metronome-note>.
   This is not yet implemented.

Lilypond does not support parenthesized tempo marks, either, so in
this case, I also generate markup, which unfortunately does not have any
effect on midi..