Dive edit: when asked ti quit while editing, offer to discard current edit

This is equivalent behavior to what we do when planning a dive.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Closing the window should behave like triggering Quit

It seems odd to have different behavior depending on whether the window
manager tells us that the user wants to close the window or whether the
user hits quit or ctrl-q.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>