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/Gui/ViewProviderDocumentObject.h

@@ -92,6 +92,9 @@ public:
     void updateView();
     /// Get the object of this ViewProvider object
     App::DocumentObject *getObject() const {return pcObject;}
+    /// Get the object of this ViewProvider object as specified type
+    template <class T>
+    T *getObject() const { return dynamic_cast<T*>(pcObject); }
     /// Asks the view provider if the given object can be deleted.
     bool canDelete(App::DocumentObject* obj) const override;
     /// Ask the view provider if it accepts object deletions while in edit