the writer drops tuplet position, `midi-unpitched` and accidental marks

[webern]

These are read into mx::api but never written back:

• TupletStart::positionData: read at src/private/mx/impl/TupletReader.cpp:28, but the tuplet
  writer (NotationsWriter.cpp:215-276) sets no position attributes. Done in fix: write back tuplet position data, midi-unpitched and accidental marks #462.

• MidiData::unpitched: read at PartReader.cpp:348, but PartWriter.cpp never writes
  <midi-unpitched>. Done in fix: write back tuplet position data, midi-unpitched and accidental marks #462.

• MarkType::accidentalMark*: read from <notations> (NoteFunctions.cpp:297-299) and from
  <ornaments> (OrnamentsFunctions.cpp:86-89). The mark loop in NotationsWriter.cpp:304 has no
  branch for them (isMarkOrnament does not include them), and
  Converter::convertAccidentalMark(api::MarkType) has no caller. The two sources also read to the
  same MarkType, so a writer could not tell which one to write.

  fix: write back tuplet position data, midi-unpitched and accidental marks #462 writes them, but only at the notations level: the api still records no source, so an
  accidental mark read from <ornaments> cannot be written back there. That needs an api-visible
  distinction plus writer grouping (an <ornaments> group must lead with an ornament), a
  public-model design pass rather than a writer branch.

Related: #403. Found while auditing silent recoveries for #432 (#439).