add auto-saves when you switch files

[JonJagger]

When doing a review there is often a change to both a code file and a test file.
It would be nice in this case (and others) to know which file was edited first.
So, the idea is that when the current file is switched, and there is change,
a (non-test) save is done.
There would be some a toggle in the UX to show these inter-test events when desired,
perhaps as a row of code/test icons below the row of traffic-lights?