gui: make plot windows children of the node, so they close automatically

Change-Id: I025bff02bc566be8bbaa8d90bf0035d1e2bf2a69
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/30494
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Georg Brandl <g.brandl@fz-juelich.de>

frappy/gui/nodewidget.py

@@ -273,7 +273,7 @@ class NodeWidget(QWidget):
             plot.addCurve(self._node, module, param)
             plot.setCurveColor(module, param, Colors.colors[len(plot.curves) % 6])
         else:
-            plot = getPlotWidget()
+            plot = getPlotWidget(self)
             plot.addCurve(self._node, module, param)
             plot.setCurveColor(module, param, Colors.colors[1])
             self._activePlots[(module, param)] = plot