Create civicrm_entity_financial_trxn records for partial payments

4.7 head does not record civicrm_entity_financial_trxn records for partial payments. These records are necessary in order to be able to allocate amounts to the financial accounts associated with line items. Prior to partial payment work, the allocation was not an issue, and every civicrm_entity_financial_trxn.amount was for the full amount of line_item.

As a default, it would be nice to have the amounts allocated proportionally to line items based on the proportion of the outstanding total balance that is outstanding for the line item. This isn't convenient to calculate. But even calculating the outstanding balance on a line_item is not much simpler, so we might as well as do it rather than arbitrarily choosing line items to pay fully.