Draft: endSessionDialog: Support eos-updater

This morning I remembered that Shell knows how to adjust its shutdown and restart dialogs if there are pending PackageKit updates. eos-updater is used in Endless OS and GNOME OS. It would be nice to support the same thing for eos-updater.

Much more invasive changes will be needed than are in this patch since the eos-updater state machine looks very different to the packagekit state machine. The update is staged in the background; once staged, the next clean shutdown/startup or reboot will boot straight into the new version. There is no distinction between an update and an upgrade; and no need/support for a yes/no checkbox.

This is purely a proof-of-concept. I do not plan to work on this further at present. I won't be offended if the MR is closed outright. Equally I'd be delighted if someone wanted to pick it up!