Summary Multiplot: drag/drop updates (#8692)

Drag'n'drop improvements - accept drops in empty multiplot areas to create new plots

ApplicationLibCode/ProjectDataModel/Summary/RimSummaryMultiPlot.h

@@ -24,6 +24,7 @@
 #include "cafPdmChildField.h"
 #include "cafPdmObject.h"
 #include "cafPdmPtrArrayField.h"
+#include "cafSignal.h"
 
 #include <vector>
 
@@ -56,11 +57,14 @@ public:
 
     void addPlot( RimPlot* plot ) override;
     void insertPlot( RimPlot* plot, size_t index ) override;
+    void removePlot( RimPlot* plot ) override;
 
     std::vector<caf::PdmFieldHandle*> fieldsToShowInToolbar();
 
     void syncAxisRanges();
 
+    void addPlot( const std::vector<caf::PdmObjectHandle*>& objects );
+
 protected:
     bool handleGlobalKeyEvent( QKeyEvent* keyEvent ) override;
     bool handleGlobalWheelEvent( QWheelEvent* wheelEvent ) override;