#12720 Histogram: Accept summary vector drag'n'drop on histogram multi plot

Add a virtual RimMultiPlot::handleDroppedObjects so RiuMultiPlotBook routes drop
events to any multi plot type instead of only RimSummaryMultiPlot. Implement the
handler in RimHistogramMultiPlot: each dropped ensemble summary vector, ensemble
parameter or file set ensemble creates its own new histogram subplot, following
the behavior of the summary multi plot. Extract RimHistogramPlot::isDroppableObject
so unsupported dropped objects do not create empty subplots.

ApplicationLibCode/ProjectDataModel/Histogram/RimHistogramPlot.h

@@ -146,6 +146,9 @@ public:
 
     std::vector<RimHistogramCurve*> histogramCurves() const;
 
+    static bool isDroppableObject( caf::PdmObjectHandle* object );
+    void        handleDroppedObjects( const std::vector<caf::PdmObjectHandle*>& objects ) override;
+
 public:
     // RimViewWindow overrides
     void deleteViewWidget() override;
@@ -177,8 +180,6 @@ protected:
     void defineUiOrdering( QString uiConfigName, caf::PdmUiOrdering& uiOrdering ) override;
     void onLoadDataAndUpdate() override;
 
-    void handleDroppedObjects( const std::vector<caf::PdmObjectHandle*>& objects ) override;
-
     void initAfterRead() override;
 
 private slots: