Log Message:
qwt6-qt5: update to 6.1.5

Maintenance
- Avoid using some of the deprecated Qt APIs
- Missing includes of QPainterPath ( Qt 5.15 ) added
- QwtPlotSpectroCurve: missing public inheritance
- QwtPlotTradingCurve: missing public inheritance

Bug fixes
- QwtLinearColorMap: handling of alpha values in color stops fixed
- QwtPlotRasterItem: missing color table initialization for
  QImage::Format_Indexed8
- QwtScaleWidget: missing QEvent::LayoutRequest added
- QwtSetSample: bound rectangle fixed for empty sets