Interview copy dataset

[jochenklar]

This PR adds the possibility to copy tabs (i.e. datasets) or better to create a new tab with all values from the current tab. It works a bit like deleting a set. If a set_value is present (e.g. project/dataset/id), the catalog is used to get all values for the current set and copy them, along with a new set_value. For # style sets without attributes this is done in the front end. There is some black magic involved when the set_prefix of values is updated with the set_index of the new set.