Added IDisabledTrait & UpgradableTrait with upgrade levels support

 * IDisabledTrait for identifying (and filtering) disabled traits
 * UpgradeEnabledDisabledTrait provides an abstract base for enabling by upgrade and disabling by upgrade
This base class provides a basic framework for traits that want simple upgrade behaviour, but we still have `IUpgradable` for things that need more complicated logic.  I don't really like the idea of adding a back-door that allows traits to ignore the *Info keys which form our primary modding documentation.