Handle orphaned documents when a taxed AP/AR Open Item is cancelled

Handle orphaned documents when a taxed AP/AR Open Item is cancelled.  Clear database entry.
The `prefix_xt*role` and `prefix_qt*role` columns provide hints to the XTreeWidget in the Qt client on formatting the `prefix` column when it gets displayed. In this particular query, the `poitem_unitprice` column should be formatted as a purchase price as opposed to a general monetary value - the two might have different degrees of precision.

While most queries will have constants for these roles, some queries might calculate values or use different values in the various parts of `union`s.