Part: add viewprovider icon for the Part::Spline object

The `Part::Spline` object is the parent of the parametric
Surface Workbench objects.

For example, the `Part::Spline` object is created by the
`Surface_CurveOnMesh` tool, which in reality just calls
the `MeshPart_CurveOnMesh` tool.

src/Mod/Part/Gui/ViewProviderSpline.cpp

@@ -51,6 +51,7 @@
 #include <App/PropertyStandard.h>
 #include <Mod/Part/App/PartFeature.h>
 #include <Gui/ActionFunction.h>
+#include <Gui/BitmapFactory.h>
 #include "SoFCShapeObject.h"
 #include "ViewProviderSpline.h"
 
@@ -71,6 +72,11 @@ ViewProviderSpline::~ViewProviderSpline()
 {
 }
 
+QIcon ViewProviderSpline::getIcon(void) const
+{
+    return Gui::BitmapFactory().pixmap("Part_Spline_Parametric");
+}
+
 void ViewProviderSpline::setupContextMenu(QMenu* menu, QObject* receiver, const char* member)
 {
     ViewProviderPartExt::setupContextMenu(menu, receiver, member);

src/Mod/Part/Gui/ViewProviderSpline.h

@@ -20,8 +20,6 @@
  *                                                                         *
  ***************************************************************************/
 
-
-
 #ifndef PARTGUI_VIEWPROVIDERPARTSPLINE_H
 #define PARTGUI_VIEWPROVIDERPARTSPLINE_H
 
@@ -43,6 +41,7 @@ class PartGuiExport ViewProviderSpline : public ViewProviderPartExt
     // Display properties
     App::PropertyBool ControlPoints;
 
+    QIcon getIcon(void) const;
     void updateData(const App::Property* prop);
     void setupContextMenu(QMenu* menu, QObject* receiver, const char* member);