Figure out if the user changed anything when editing

And mark the divelist changed.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Show the correct question at exit when there are unsaved changes

We want to give the user the option to 'cancel' and not exit the program,
to 'save' the file, or to say I'm 'OK' with losing the unsaved data.

This does NOT implement the actual save / save-as, yet.

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