packagekit: Auto-prepare update when the file is deleted

The PackageKit prepared update can be deleted in some cases, sometimes as a side-effect of some actions done by the Software itself, like when calling refresh after start of the application. Due to the PackageKit can be used by other applications as well, listen for the changes on the prepared update file and restore it in case it disappears.

The auto-prepare of the update is triggered only if the file was deleted and the user has set to download updates. It's not triggered when the Software is started and there is no prepared update. It's because the file name is not exposed in the public PackageKit API.