use specific Qt settings group for this interface, re #12610

Code/Mantid/MantidQt/CustomInterfaces/inc/MantidQtCustomInterfaces/EnggDiffraction/EnggDiffractionViewQtGUI.h

@@ -136,7 +136,7 @@ private:
   virtual void closeEvent(QCloseEvent *ev);
 
   // path/name for the persistent settings group of this interface
-  std::string m_settingsGroup;
+  const static std::string m_settingsGroup;
 
   // here the view puts messages before notifying the presenter to show them
   std::vector<std::string> m_logMsgs;

Code/Mantid/MantidQt/CustomInterfaces/src/EnggDiffraction/EnggDiffractionViewQtGUI.cpp

@@ -40,6 +40,8 @@ const std::string EnggDiffractionViewQtGUI::g_pixelCalibExt =
     "(*.csv *.nxs *.nexus);;"
     "Other extensions/all files (*.*)";
 
+const std::string EnggDiffractionViewQtGUI::m_settingsGroup = "CustomInterfaces/EnggDiffractionView";
+
 /**
  * Default constructor.
  *