NAB: Allow same file to be defined more than once

[daveo]

When creating a NAB data source using the wizard there are occasions when it would be helpful to be able to add the same file more than once. For example you may need to list transactions and each transaction has a submitter and an approver so you need to get the description of both of those users, from the same file but using a different field to join.

I am aware it can be done via a SQL statement but it would also be useful in the wizard.