ValuesClass was missing from *Accessible

it was changed from attribute to column
any false ValuesClass should be valid

RT::CustomField->ValidateValuesClass must allow "0" as
RT::Record->_Set replace undef value by 0
Fix CF ValuesClass update in web UI

* update only if needed
* display localized update result
tests around RT::CustomField->SetValuesClass

UI and API
Merge branch '4.0/fix-cf-valuesclass-updates' into 4.0-trunk