Undo editor change to LightProbeGroup.probePositions

I wrote an editor window to help with creating light probes. However, I have been unable to properly register Undo events for this. Undo.RecordObject has not solved the problem.

This is probably because the way you add/remove light probes to a group is just by changing a public property, and properties probably aren't recorded as part of the undo recording.

I'm not sure how to create custom undo events, and have been unable to find any information on this, although I would like to imagine that Unity includes this capability.