SLING-4161 support Sling Validation by a new field on the Model annotation

The dependency to Sling Validation is optional (i.e. Sling Models still work even if  Sling Validation is not deployed)

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1644069 13f79535-47bb-0310-9956-ffa450edef68