QA/2d customise symbol color

Let the user define the color used for the symbols on the PfPv
and Scatter plot when framing intervals.

src/ScatterPlot.cpp

@@ -218,7 +218,7 @@ void ScatterPlot::setData (ScatterSettings *settings)
     sym.setPen(GCColor::invert(GColor(CPLOTBACKGROUND)));
     sym.setBrush(QBrush(Qt::NoBrush));
     QPen p;
-    p.setColor(Qt::gray);
+    p.setColor(GColor(CPLOTSYMBOL));
     sym.setPen(p);
 
     // wipe away existing