Gui: Add getObject<T>() helpers to various classes

Obtaining specific kind of document object is very common task. This
commit introduces handy helper which makes that a lot easier.

src/Mod/Fem/Gui/ViewProviderFemPostFunction.h

@@ -65,10 +65,15 @@ protected:
     {
         return m_view;
     }
-    Fem::FemPostFunction* getObject()
+    Fem::FemPostFunction* getObject() const
     {
         return m_object;
     }
+    template<class T>
+    T* getObject() const
+    {
+        return dynamic_cast<T*>(getObject());
+    }
 
     bool blockObjectUpdates()
     {