Interestingly, this moves the filename input's cursor to the start,
whereas it was positioned at the end when using the timer.
Unfortunately the Input control doesn't support selection,
which is what I would really want as the behavior.
- Confirm discarding changes for Open, New, or Exit, including for
exit via Ctrl+C which was previously handled by a built-in binding.
- Await Save As dialog closing, including when Save triggers Save As.
This is my first time using asynchronous features in Python,
(as far as I remember,) so it's a bit messy.
- Make DialogWindow callback also for Cancel, which means all
DialogWindow usage sites care what button is selected.
- Send RequestClose event for Esc key.
