[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.

[user management] users and groups management now really lead to that (closes #1931478) ie, one tab for users, another for groups. By default the groups tab simply list existing groups (with the number of users they count), and propose to create a new group.