Manage offline updates over D-Bus interface

[mcrha]

First of all, it's likely I overlooked the API in the D-Bus API definition, I'm sorry about that if I did. What I'm looking for:

A PackageKit-based distro with gnome-shell can ask the user when logging-out/rebooting whether the user wants to install pending updates. The PackageKit can prepare such updates under the hood, and depending on the action chosen by the user the update may or may not be scheduled for the reboot and the machine powered off or rebooted after the update is installed. It's why it's called offline update, because it's done shortly after boot.

The gnome-shell needs for this only three functions:

1. get state - returns whether the update is or is not scheduled on the reboot (it can return a text, rather than bool, in case of expanding it to more states in the future)
2. cancel - cancels any scheduled update
3. set action - sets what to do after the update is finished, like reboot or shutdown

I do not expect direct functions for these, the set action might not even be applicable here, because the updates work differently, but I hoped for the first two there could be something, I only did not see it. I'm not much familiar with the terminology, which does not help.

Related proposed change for the gnome-software: https://gitlab.gnome.org/GNOME/gnome-software/-/merge_requests/2307 to make it easier for the gnome-shell (there's a link to the gnome-shell bug).