Fix #27201 - PLUGINS: methods returning pntrs to QObject lead to crash.

Fix #27201 - PLUGINS: methods returning pntrs to QObject lead to crash.

For details, including an analysis of the problem, see http://musescore.org/en/node/27201

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)
- **Note**:
    - accidental
    - dots[]
    - tieFor
    - tieBack
- **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.