Fix selection logic when manually adding a dive

This was an interesting bug. When adding a dive that would end up in the
middle of the dive list, the newest dive in the dive list would end up
marked in the dive structure as selected - even though it wasn't
visualized as selected by Qt. Bad things happen if the user then made
changes to that dive without selecting something else first, for example
by either editing the dive or doing things to it like removing it from or
adding it to a trip. The same operation would also be applied to the
newest dive in the dive list.

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