Update total when cell is edited

Some SellableItemStep's children will set some columns as editable and when
that value is changed, they would need to call self._update_total (or a
duplication of that code, since it's private) by hand.

Instead, connect on the event and call it for them.

Change-Id: Ie232a461b843ef376a91774f9b960309888f204b
Merge "Update total when cell is edited"