Send notification about adding to cart in Witnesses.add_operation_to_cart...

Send notification about adding to cart in Witnesses.add_operation_to_cart instead of in the GovernanceActions.add_row method

- counting votes can be made by analyzing cart instead of storing
  counter and incrementing/decrementing everytime a row is added or
  removed
- removed unused _action_votes member and hook_on_row_added method