Fixes the following Q_INVOKABLE methods returning a QObject* by turning them into a property:

- Measure:
-- firstSegment
-- lastSegment
- MeasureBase:
-- nextMeasure
-- nextMeasureMM (new)
-- prevMeasure
-- prevMeasureMM (new)
- Score:
-- firstMeasure
-- firstMeasureMM (new)
-- (for firstSegment(), see special cases below)
-- lastMeasure
-- lastMeasureMM (new)
-- lastSegment

- Segment:
-- next (renamed from `next1`)
-- nextInMeasure (renamed from `next`)
-- prev (renamed from `prev1`)
-- prevInMeasure (renamed from prev)

Special cases:

- Cursor: The prototype of the `Q_INVOKABLE Ms::Note* Cursor::addNote(int pitch)` was wrong: corrected in `Q_INVOKABLE void Cursor::addNote(int pitch)`.
- QmlPlugin: `Q_INVOKABLE Score* QmlPlugin::readScore()` and `Q_INVOKABLE Score* QmlPlugin::newScore()` has been kept, as they are intended to be called from QML; code has been added to ensure the C++ ownership of the returned object.
- Score: `Q_INVOKABLE Segment* Score::firstSegment(Segment::Type segType)` is kept (as it needs a parameters), but code is added to ensure C++ ownership of the returned Segment*.
- Segment: `Ms::Element* Segment::element(int track)` has been made NOT Q_INVOKABLE; a variant `Q_INVOKABLE Ms::Element* elementAt(int track)` has been added specifically for QML with code to ensure the C++ ownership of the returned Element* (this was the cause for the crash of the Walk plug-in).
- FiguredBass: `Q_INVOKABLE Ms::FiguredBassItem* FiguredBass::addItem()` has been removed; plugin interface for FiguredBass needs to be redesigned anyway.

The few occurrences in the supplied plug-ins of the methods whose names did change have been updated.