fix: bump overlay retained segs, remove from overlay if all nodes deleted, invalidate history on source change

[seankmartin]

• feat: bump default fixed overlay seg limit
• fix: check for source changes for invalidation previously the layer was just checking for if the layer changed away from having spatial skels. But if you change the source, then it never knew and would render stale data and have a stale history which doesn't apply to the source.
• fix: when removing all nodes from a seg, remove in overlay retained seg

[seankmartin]

Leaving in draft as gives as an idea for the sources, but overall you could have multiple catmaid sources on one layer so I feel like something is a bit not quite right there with that idea. I get the sense that the core problem is that there might be only one overlay, and then each source is putting into the same overlay.

If there is an overlay per source then it might more naturally solve this invalidation. There's of course the related issue of what you are editing in such a setup.

I believe the retained overlay segment fix should be a pretty harmless one though, unlikely to come up too often however so not a big deal