Add support for optional Livetable columns

It would be useful for example in the Document Index Livetable to display other document metadata, like parent document, creator, creation date, language. Displaying all these fields by default isn't good, since it will create very cramped columns, the table would probably overflow the content column, and, most importantly, all this information is going to suffocate normal users. So, it would be nice to be able to configure the Livetable so that it knows about all these extra columns, and the user can select which extra columns to be displayed.