Add a generic way to declare validators on builtin widget types

Most of standard widget types are using standard JSF components, that accept
standard validators. There are two ways to declare a validator on a JSF component:

• use the "validator" property to declare a method binding that will perform the
  check and throw a ValidatorException error if validation fails. This is already
  available.
• add a tag inside the component to declare another validator. For instance:
  <h:inputText value="# {field_0}

  ">
  <f:validateLongRange minimum="1"/>
  </h:inputText>

The "f:validateLongRange" is a standard validator. The "f:validator" tag also makes
it possible to declare a validator by id.

Custom validators can also be added to the framework. For instance, ui-web defines
validators "nxu:validateFileSize" and "nxu:validateFileMimetype" that can be used
inside the "nxu:inputFile" tag.

As these tags all accept specific attributes, maybe a generic way to declare them on
the resulting component would be needed.

Another idea would consist in defining validation rules on the layout, so that condi-
tions can be easily expressed in a el-like language, with some variables exposed (to
define.

Syntax is still to discuss.