[sources management] properly set __linkto, closes #1922970
when clicking on 'add mapping part' on an external source view, one expect to have source properly selected in form
[table view] allow to set None in 'headers', meaning the label should be fetched from the result set as done by default (closes #1931533)
the point of this is to avoid having to define the whole headers list when only
some of them have to be defined.