Use the async file loading/saving features in NSDocument, and remove PPApplication

[choll]

Recall that PPApplication only exists to prevent the application quitting while an async operation is running, as the async NSDocument features didn't exist when it was written.

[choll]

Fix issue 13: Remove old background saving code, replace with NSDocument background saving feature, also display saving indicator instead of a progress bar.

→ <<cset a4feeaec117f>>