QwtPlotItem to emit a new signal when legend has changed

[CEAVirginie]

emit a Signal if legend changed (QwtPlotItem must derive from QObject)

[PierreRaybaut]

Two things are bothering me for this one:

1. QwtPlotItem object does not inherit from QObject as of today because it was designed to be a neutral object with respect to Qt.
2. There is already a signal emitted in this situation: QwtPlot.legendDataChanged. So, providing a minor change of implementation in the code using PythonQwt, this use case is already covered by PythonQwt