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
Not a blocker, but it might be a good idea to add a comment `// Disable the actor when this trait is enabled` to avoid confusion.  I did a double take when I saw this, and had to think about what it was actually doing.