Validators

At present we have some fields validation features, but I think we need to systematize it. What you think about validators in Propel. We could have additional table with "name", "value" and "message" fields, just like for every validator in Propel. And then, every row of this table will be related to every defined field type. Moreover, some validators will be specific to text, number, decimal, nodereference, userreference, etc. This feature will tie currently idea together -- I mean allowed values, regular expression matching, max/min length, noderefrence to witch content type etc.