XY-Plot setup handling improved

- Use readable names for axis type/mode in json (old integer encoding still valid when loading old setups)
- Bugfix: time domain traces not included in context menu when plot is constructed after the trace

Software/PC_Application/Traces/xyplotaxisdialog.cpp

@@ -183,6 +183,8 @@ std::set<TraceXYPlot::YAxisType> XYplotAxisDialog::supportedYAxis(TraceXYPlot::X
         ret.insert(TraceXYPlot::YAxisType::Step);
         ret.insert(TraceXYPlot::YAxisType::Impedance);
         break;
+    default:
+        break;
     }
     return ret;
 }