Support automatic and custom origins with a sentinel value instead of the clunky originOverride UI.
This improves persistence of custom origins quite a bit too...anytime you start over it'll be reset to automatic, but using an existing slave (or relative) as a template will keep the custom origin from that slave. No fiddling with the override setting required.
Seems to work great in the cases I've tried, but I wouldn't mind if someone wanted to test it a bit harder.
