Allows update on restart or shutdown when this feature are available.

Offline updates are available since systemd 183 (May 31, 2012), however it is a feature that of course is only used by Gnome.

See:

• https://freedesktop.org/wiki/Software/systemd/SystemUpdates/
• https://github.com/PackageKit/PackageKit/blob/main/docs/offline-updates.txt

The updates are downloaded by PackageKit (pkcon --only-download update or gnome-software), and here we only detect that there are updates available to change the buttons from Shutdown and Restart, to Update and Shutdown, and Restart and update respectively.

Next, we must tell PackageKit that we want to apply the updates when appropriate.

If PackageKit is not installed or systemd is not used, it does not change any behavior. On the other hand, if the updates are not downloaded with any of these applications, the behavior does not change either.

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Closes: #71 (closed)