Added

• add optional [validate_{slot}]{.title-ref} methods to [FormAction]{.title-ref}
• forms can now be deactivated during the validation function by returning
  [self.deactivate()]{.title-ref}
• Function to get latest input channel from the tracker with
  tracker.get_latest_input_channel()

Changed

• self._deactivate() method from the FormAction class has been
  renamed to self.deactivate()