Blender Git Loki

Fix T79987: Crash when joining objects

Mistake in b077de086e14. I did the same fix for a few operators there,
but missed the object "Join" one.

The joining operator changes the layer content. So it must send a
notifier for that.
Before b077de086e14 that didn't cause a noticeable issue, because the
Outliner happened to listen to other notifiers (active/selection
changes) the operator sent and fully rebuilt its tree in response. Now
missing these notifiers can be more problematic, since we try to avoid
more rebuilds.

Added comments to the notifier types to avoid at least this pitfall.