MusicXML: Fix issues with dynamics attached to notes (not measure position)

MusicXML: Fix issues with dynamics attached to notes (not measure position)

-) Implement the missing musicxml_dynamics_to_lily_event function (simply
   moving code around!), so that dynamics attached to a single note (as
   opposed to given in a <direction> element) are also correctly converted.
-) When we convert unknown dynamics texts using \markup, we need to prepend
   "-".
-) Handle multiple <direction-type> children in a <direction> element.