Add "Unallocated Amount" info in the "Allocate detailed receipt" function

Give us "Unallocated amount" information like the one in fast coding.

When inside "Allocate detailed receipt" view, very often the payment is for multiple invoices. Currently there is no way to know which invoices should be allocated to this bulk payment. This is so inconvenient.

Give us "Unallocated amount" info as we allocate invoices to this payment will greatly speed up our work flow.