Various enhancements

[warnes]

1. Add a new colnames parameter that is passed onto the wrapped DT::renderDataTable to allow control of the column labels of the displayed table.
2. Add new arguments to control whether action buttons are enabled or disabled on startup.
   The state can then be changed later with code like:

 if(user_info()$group == 'admin') 
        {
          enable('eca_data_edit',   asis = TRUE)
          enable('eca_data_add',    asis = TRUE)
          enable('eca_data_remove', asis = TRUE)
          enable('eca_data_copy',   asis = TRUE)
        }
        else
        {
          enable ('eca_data_edit',   asis = TRUE)
          disable('eca_data_add',    asis = TRUE)
          disable('eca_data_remove', asis = TRUE)
          disable('eca_data_copy',   asis = TRUE)
        }

3. Show an explanatory message if the user clicks edit, copy or delete without selecting a row.
4. Modify the 'Delete' modal dialog to have the same appearance as the other modal dialogs, by using the same tags, with all of the data entries disabled.