Change From dates in separator lines on MBOX export

[lizfischer]

This is a feature request. Currently, Thunderbird puts the date it synced the email in the "From - " line at the start of a message. This gets interpreted by many MBOX viewers as the date received, which technically in the MBOX spec it should be, but it's a bit misleading. Other Thunderbird export tools, like Emailchemy, fix this problem by changing the "From - " line during export to include the date listed in the "Date:" field of the message header. It would be a huge help if ImportExportToolsNG had an option for this when exporting folders.