Datatable column improvements

- share/templates/crudite_config: Datatable columns whose title is the
  empty string aren't searchable or sortable; use those for columns
  with action buttons. You can have several of those; previously the
  title had to be "Actions", which looks strange if you have several
  such columns. And it's clear from the buttons in those columns that
  they produce some sort of action.
- Also, datatable columns whose title is '*hidden' are hidden.