Handle redirection inside the cancel method in order to allow developers...

[airtonix]

... to override the redirection target.

Currently the only way to override redirection target in the 'cancel' scenario is to suffix your cancel link with ?rd=/some-path/ which is fairly subpar.

This patch changes nothing of the default behaviour, except that it now allows subclassed wizards to redirect to a location by specifying it inside the wizard python code.