Change EDIT_SELECTED_DIVES macro

The way this is implemented is broken in several ways.
This fixes the first issue.
For the invocations where we are in the 'WHAT' checking to see if the
value we are changing in the selected dive was previously the same as in
the current dive (which is the one shown to the user for editing), then we
need to make sure we change the current dive last, otherwise the
comparison will fail.

Of course, right now we only do this check for gps location, which is a
massive bug as far as I am concerned.

Fixes #515

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Only change identical fields when editing multiple dives

This was broken when porting to Qt - we used to do this correctly in the
Gtk version.

When editing multiple dives we show the current dive to the user and allow
them to edit that and then apply those edits to all selected dives. The
way this is SUPPOSED to work is that we only change those selected dives
that had the same value for the edited field as the current_dive had for
that field.

Let's say you select ten dives. The current dive shows divemaster Joe. You
change that to divemaster Jim. Then only the selected dives that had
divemaster Joe should change to Jim. All other dives should stay
unchanged.

This seems to implement that logic.

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