[FEM] fix missing colorbar update for FemPostObject

- fixes issue #7230

- to update the color bar for post objects, the Field property has to be reset to trigger this. This PR does this.

src/Mod/Fem/Gui/ViewProviderFemPostObject.h

@@ -54,6 +54,7 @@ class SoIndexedLineSet;
 class SoIndexedTriangleStripSet;
 
 namespace Gui {
+  class SelectionChanges;
   class SoFCColorBar;
 }
 
@@ -145,6 +146,10 @@ protected:
     vtkSmartPointer<vtkExtractEdges>            m_wireframe, m_wireframeSurface;
     vtkSmartPointer<vtkVertexGlyphFilter>       m_points, m_pointsSurface;
 
+    void selectionChanged(const Gui::SelectionChanges &);
+    typedef boost::signals2::scoped_connection Connection;
+    Connection connectSelection;
+
 private:
     void updateProperties();
     void update3D();