Fix ESC key handling in Save dialog.

Closing unsaved dive raises Save dialog with Yes/No/Cancel buttons.
Pressing ESC dismisses dialog without saving changes.

This commit adds GTK_RESPONSE_DELETE_EVENT response code check in order
to fix that behavior.

Reported-by: Jan Schubert <Jan.Schubert@GMX.li>
Signed-off-by: Sergey Starosek <sergey.starosek@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>