unlock-dialog: Do not allow triggering shutdown action

[Deminder]

Since https://github.com/Deminder/ShutdownTimer/pull/9 the indicator and quick settings item are visible during unlock-dialog (without quick acess to openPreferences).

However, during unlock-dialog GNOME Shell hides shutdown options from unauthorized users and only shows Suspend. In order to not contradict this, the ShutdownTimer should also not allow initiating the destruction of the user session by shutdown.

Solution

Hide the ShutdownTimer Quick Settings item during unlock-dialog.