Add support for more merge data sources (ODS, Database table)

[Jérémie Tarot]

It would be awesome to get data directly from

• ODS spreadsheets sheets, with a drop down menu to select the desired sheet
• Database table, either specifying the table in a field or, here also, with a drop down menu to select database table

This would avoid the burden of exporting sheets to CSV/TSV as well as the risk of desynchronisation between merge source data and original database