Implement "Exit without saving" prompt.

[Sintrastes]

As we move to a more detailed route-based navigation system, we will need to abandon our current free-monad approach to building a tab switcher UI.

This will require persisting our schema data to disk before leaving a page so it can be read from disk elsewhere in the app.

When we implement this, the user should be prompted as to whether or not they want to save their unsaved changes (if they have any)