FEM: Fix linking errors

The static members of FemSetElementNodesObject are not defined in Fem but FemGui which the MSVC compiler doesn't like.
Furthermore it's a code smell to make them public and static and they are not even used by the class itself.

src/Mod/Fem/App/FemSetElementNodesObject.h

@@ -53,8 +53,10 @@ public:
     }
     short mustExecute() const override;
     PyObject* getPyObject() override;
-    static std::string elementsName;        //  = "ElementsSet";
-    static std::string uniqueElementsName;  // "ElementsSet" latest name
+    static std::string getElementName()
+    {
+        return "ElementsSet";
+    }
 };
 
 }  // namespace Fem