MainWindow: replace stack with Deck

[danirabbit]

This makes a couple of assumptions that need to be verified:

• [ ] It's always okay to go back except on progress view and success view. We may want to either disable navigation or make corrections on some pages.
• [ ] navigation is always linear. It's possible the old code set something as previous_view that wasn't actually the previous view. We should make sure views self-destruct if they can't be returned to after navigating away.
• [ ] Every child is an AbstractInstallerView. If any aren't, they'll need additional margins