Turn the Upgrader frontend into a wizard
... just like the persistent volume assistant. The "steps" logic from t-p-s could likely be factored out and reused.
Rationale: the current UI flow (based on zenity dialog boxes) could be clearer if merged into a single window. Otherwise, it's not that clear that all these dialog boxes are part of the same process, and it's far too easy to close one and have no progress information until the next dialog box appears, while the upgrade process is still ongoing in the background.