Improve syntaxe for more flexibibility: callbacks, fields operations, no field

[jclaveau]

Possibles API examples:

[function($row) {return $row->getField1();}, 'regexp', "/^prefix-[^-]+-suffix$/"],
[function($row, $params) {return $row['field_1'] == 3;}, [param1, $param2]],
['field_1', 'apply', "is_numeric"],
['field_1', "is_numeric"],
['field_1', 'apply', [$this, "method"]],

This kind of syntax could be interesting also:

['field_1 + field_2', '=', 3],

Consider a syntax without field for one dimension array:

['=', 3],
['row', '=', 3],