Return HTML form element for the search operator for the specified column and type
ns_searchopwidget returns the HTML form element for the operator to be used when matching the column.
For numeric types, the operators in the form element are "=", "<", ">", "<=", ">=", "<>", "is null", and "is not null".
For alphanumeric types, the operators are "=", "<>", "contains", "like", "not like", "is null", and "is not null".
For all other types, including boolean, the operators are "=", "<>", "is null" and "is not null".