Model validations - client and server side

[pvlasov]

In addition to form validators annotations introduce org.nasdanika.cdo.web.validator annotation which supports the following keys:

• client - synchronous validation on the client. For properties this validation will be performed in setter functions and throw an error if it fails.
• client-async - asynchronous validation to be performed in $store() or $apply()
• server - validation on the server side.

The validator annotation shall be applicable to EClasses, features, EOperations and EParameters.

On the server side use Rhino JavaScript engine to execute validation logic. If validation fails, use OK return code, but { error: ... } response. Modify session unmarshal logic to throw an error if the response has error key.