The block-based API returns a token that needs to be explicitly
unregistered, *and* the block should not capture 'self' strongly.
The fixes in this PR come in three flavors:
1. Switch to the selector-based API, sometimes taking advantage of
the fact that extra arguments to an Objective-C method can be
safely discarded (because they are passed at +0).
2. Save the observer token and unregister in deinit.
3. Comment why it's safe to leave the observer registered forever
in this particular case.
4f06a6d8a5