Allow the use of an offset for the numbers automatically generated in the names of documents which have an empty page name

Currently, the page name generated for the lines which have an empty page name (e.g. not mapped or value in the mapped column is empty) is generated as follows:

• the "empty doc name prefix" configuration is used as a prefix (is usually one or multiple letters)
• this prefix is padded with the row index of the line being imported.

This issue is about being able to use a numeric offset for this row number, which will be added to the row index and the numeric result thus obtained would be padded to the empty doc name prefix.
This could allow executing multiple imports with empty page names, from multiple files, with continuous numbers (because the last number from the previous one will be added as offset for the next one).