UI restructure: cleanup the code when canceling an edit

This was way too complicated because we used to edit a "live" dive that
was on the dive list. All we really need to do is systematically make
sure that all the widgets are in the correct state.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
UI restructure: remove the backupDive we no longer need

Since we have a dedicated dive that we use to display things, we can
simply get the information from the current dive and use it to setup said
displayed_dive.

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