SignalXY: add option for step display

SignanXY() in Ver5.0 doesn't have Stepline property

**ScottPlot Version: 5.0.32**

**Code Sample:** 
```cs
ScottPlot.Plot myPlot = new();
/* code to reproduce the bug */
  var signal = scottPlotUC.Plot.Add.SignalXY(xData, y1Data);
signal.Stepline = true;  (this property not exists but it exists in Ver 4.0)
```