That's what this class really is; in fact that's what the first line of
the comment says it is.
This commit does not rename the main files, since those will contain
other time-related classes in a little bit.
Not a single client of this API actually used the event mask feature to
listen for readability AND writability.
Let's simplify the API and have only one hook: on_activation.
The macOS FileWatcher depends on macOS dispatch queues, which run on a
different thread than the Core::EventLoop. This implementation handles
filesystem events on its dispatch queue, then forwards the event back to
the main Core::EventLoop for notifying the FileWatcher owner.