The receiver has just appeared in a new world. Note:
* aWorld can be nil (due to optimizations in other places)
* owner is already set
* owner's submorphs may not include receiver yet.
Important: Keep this method fast - it is run whenever morphs are added.
If we're in the business of removing pauseTick controls when we're neither paused nor ticking, then do it now. The present take is not to remove these controls, which explains why the body of this method is currently commented out.