Rather than having a CompletionView instantiate and register a
Completer, instantiate both in MainWindow. The CompletionView is the
parent of the Completer, and communicates by emitting
selection_changed, meaning it no longer needs to contain a reference to
the Completer.
- clean up docstring typos
- use _ to name an unused loop variable
- parent the filter model to avoid an issue with disposal
- use mocker.patch instead of monkeypatch to mock Completer creation
- use is instead of == to compare by identity