(Not meant to cause user visible changes.)
Makes use of the new iterators introduced in the previous commit. Some
benefits:
- Shorter, simpler, easier to read & understand
- Deduplicates logic
- Centralizes iteration logic, making it easier to replace tree storage
(as planned), see previous commit.
- Avoids having to pass (sub-)tree to iterate around (often redundant
since it's just `SpaceOutliner.tree`, even though `SpaceOutliner` is
already passed).
- Function arguments that are only passed to the recursive call are
recognized as unused (found and removed a few).
Also does some general cleanups while refactoring the code for the
iterators. Use `const`, use references (signals null is not expected),
early-exit (see 16fd5fa656), remove redundant arguments, etc.
dc6fe73e70