Export some functions and properties for use in plugins

This exports the following to the qml framework:
Part::startTrack, Part::endTrack, Score::parts
- to be able to identify the staves belonging to the same part.

Score::changeAccidental(Note*,Ms::Accidental::Type)
- to be able to add courtesy accidentals

Segment::tick
- to be able to parse several tracks simultaneously, because a cursor always proceeds to the next element in a given track.