Segment override values can be persisted without explicit save.

If I edit a feature, change a segment override, then go to the “value” tab without saving the segment override first, change the flag’s value and save that change, when I return to the “segment overrides” tab the new value is replaced with the old value (but the “unsaved changes” marker is still present).

Either:
- Don’t replace the new, unsaved value with the old value.
- Or, have a single save for the whole drawer (which I think is clearer overall and prevents errors like this).