RFE: allow unlocking all documents to be merged at once

[AppleBottom]

I occasionally have to merge a fairly large number of individual documents, all of which are password-protected with the same password. Currently, this requires clicking on the "Lock" icon and entering the password for each individual document. It would be nice to have a global "Lock" icon in the corresponding column header, allowing the user to enter the same password, used to unlock all documents, just once.