Remove temporary authorization when the subject it applies to vanishes
This makes it easier to write the desktop component showing a notification icon - said component now only needs to watch ::changed and reenumerate temporary authorizations. If this is done, then the notification icon is updated in near-realtime. Also emit ::changed on ConsoleKit changes. This helps remind Mechanisms that they should redo an authorization check (if this is how the Mechanism decides to cache authorizations).
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