Added IDisabledTrait & UpgradeManagedTrait

 * IDisabledTrait for identifying (and filtering) disabled traits
 * UpgradeEnabledDisabledTrait provides an abstract base for enabling by upgrade and disabling by upgrade
The Desc attribute isn't very useful here as these classes are not traits by themselves, and so this won't be seen by our automated documentation generation.  You may want to use a `///<summary>` instead.