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/RimHistogramMultiPlot.cpp

@@ -341,6 +341,8 @@ void RimHistogramMultiPlot::initAfterRead()
         plot->titleChanged.connect( this, &RimHistogramMultiPlot::onSubPlotChanged );
         plot->autoTitleChanged.connect( this, &RimHistogramMultiPlot::onSubPlotAutoTitleChanged );
     }
+
+    loadDataAndUpdate();
 }
 
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