Show correct path after saving dive computer settings

When backing up dive computer settings to XML, the confirmation
dialog shows the default file path. This patch fixes that by
displaying the correct file if the user changes the backup location.

Signed-off-by: Joseph W. Joshua <joejoshw@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Dive list: seriously simplify the "add to trip" code

The existing code seemed way complicated, made way too many assumptions
and apparently was broken in certain cases.

This code seems very simple, looks correct and should fail gracefully
(i.e. simply do nothing) if things get confused.

Fixes #706

(I hope)

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