Plato: Handle Disabled fields

[GoogleCodeExporter]

It is natural to have constraints that require values for certain fields.  Such 
constraints are only falsified when that field is disabled (though see HTTP 
spec for when a value is not submitted to the server).  Checking such 
constraints on the client requires analyzing DOM properties.  

Another example of such a constraint is VISIBLE. 

Add to Plato a mechanism for implementing constraints that check/change DOM 
properties.  

Original issue reported on code.google.com by [email protected] on 21 Jun 2012 at 3:31