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
One of the use cases for this trait is to clean up system actors (e.g. buildable upgrade sources) to undo their effect in certain conditions.  The IsInWorld check will break that.