Enforce that store and layer names are xml-safe at the catalog level

As discussed on the mailing list, it is problematic to have layer name that are not XML-safe in GeoServer.  This patch adds a validator to reject such names, and ensures that such validators are enforced in the REST API and web interface.  I think that it still needs a little work to give better errors (right now any validation is reported with a generic "Validation failed" message) but here is the approach.
Thinking we should provide some sort of context in terms of why validation failed. So maybe passing in the first exception as the cause? And stating something like "validation failed with n errors".