Markup isn't handled properly

[uliska]

#103 falls short of properly handling markup in two respects:

1. the exported markup is always "above" (which should be easy to fix, I think)
2. Instead of attaching markup to the note/event all markup words are concatenated to a single text and attached to the beginning of the measure.

I think 2. is harder to fix, especially since the current behaviour seems to be achieved on (some) purpose.

[uliska]

ad 1):
In <direction placement="above"> the placement attribute can simply be left out if there is "neutral" direction.

ad 2):
The <direction> has to be inserted before the note it is attached to (I think this means "before the <note> adds a duration to it").
One <direction><direction-type> element can contain multiple <words> elements, which can be used to apply changing formattings.

One more thing is: any note (=> what else can a markup to attached to? Rests, anything more? How is the relation to marks or tempo indications?) can have multiple markups attached to it (with the same or differing directions). So we have to do a number of things:

• when parsing, \markup "starts" a markup (different from the current state) and attaches it to the current note. This must be done by appending to a list of markups.
  Note that this has to know about any preceding direction operator, which has to be attached to the markup object
• each MarkupWord has to be appended to the current markup object
  (Does MarkupWord "know" about the formatting?)
• When writing the musicxml a current note has to write out the list of markups before itself.
• All references to the current measure in the existing code have to be reviewed and probably removed