data_set_clipper broken with VTK6

Hi,

Trying to use the data_set_clipper on a machine with VTK6 (either programmatically or adding the filter through the GUI) leads to errors.
After searching, it seems that in the switch to VTK6 every direct assignments like `value.input = self.inputs[0].outputs[0]` have to go through a function ie. `configure_input_data`.
Fixing this gets ride of errors, but the clipped data is never updated when moving. After a lot of trial/error it works for me to add a `self.inputs[0].data_changed = True` in interactive event handling.

As I'm no real VTK user nor mayavi developper I've no idea what the real fix would be. Since adding the filter through GUI is also broken, I don't think that my code is guilty. Maybe this filter was forgotten at VTK6 support step? But it's a very useful one :)