Add support for using direct functions on column definitions for editable, renderable & sortable

Row(model)-wise support for editable, renderable & sortable.
Analogous with sortValue.
example:
    {
      "name":"amount_order",
      "editable":function(model) {
        return model.get('status') < 2
      },
      "cell":"integer",
      "label":"Tilaus"
    }