DataSource menu: should also include the bucket

[rpbouman]

If the datasource menu has multiple items, then the union of all these items should also be represented as an item.

[rpbouman]

Alternatively, we might let the items in the datasource menu be checkbox items, and control which of the datasources are included by checking and unchecking them.

The nice thing about that is the user can fully control wich files to include.