Histogram drag/drop and paste improvements (#12851)

* Add drag'n'drop support for ensemble summary vectors and ensembles into histogram plots

* Make sure histogram plot is shown and all update signals are connected when loading a project.

* Support ensemble parameter drag'n'drop from data sources into histogram plots. 

* Show ensemble parameters in data sources tree.

* Add copy/paste of histogram curves

* Re-use all existing parameters in a plot when dragging a new ensemble into it

ApplicationLibCode/ProjectDataModel/Histogram/RimEnsembleParameterHistogramDataSource.h

@@ -37,8 +37,11 @@ public:
     RimEnsembleParameterHistogramDataSource();
     ~RimEnsembleParameterHistogramDataSource() override;
 
-    void setEnsembleParameter( const QString& ensembleParameter );
-    void setEnsemble( RimSummaryEnsemble* ensemble );
+    void    setEnsembleParameter( const QString& ensembleParameter );
+    QString ensembleParameter() const;
+
+    void                setEnsemble( RimSummaryEnsemble* ensemble );
+    RimSummaryEnsemble* ensemble() const;
 
     std::string unitNameX() const override;
     std::string unitNameY() const override;