Sessions should preserve reordered terminal tabs

[dbarnett]

What steps will reproduce the problem?

1. Start a8 and open several terminals in different directories.
2. Drag terminal tabs into a different ordering.
3. Restart a8.

What is the expected output? What do you see instead?
Tabs should reflect the latest ordering, but instead return to the order they were originally opened in.

It can be surprisingly annoying for long-running sessions with some churn, where some tabs get accidentally closed. I sometimes hand-edit the .a8/session.yaml file to fix it.