Remove temporary authorization when the subject it applies to vanishes

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).