Add support for property information helpers in vtkObjectWrapper

vtkObjectWrapper can now access properties that do not map directly to a vtkObject method (and thus vtkReflection cannot be used.) This work reuses the idea of property helpers from paraview master and takes advantage of the xml <...Helper/> tag for these properties.

For now only TimestepsHelper is implemented.

The API is really rudimentary and it will to be exteded for other helpers but since this is internal methods used only by vtkObjectWrapper we can expand it as we add more methods.