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
Should also re-run this on a clean set of rules to make sure that it actually works.  The current release has caused some problems for modders because we merged upgrade rules that were written to emulate changes that were made by hand, but which didn't work.