Another selection fix

Another selection fix

The corner cases are getting more and more artificial. Without this patch,
the following can happen:
Select one or more dives in an (expanded) dive trip. Now collapse that
trip with the little triangle. Select a different trip. The previously
selected dive(s) are still part of the selection (as you can see, for
example, in the statistics tab).

With this patch the scenario above works as intended (all the dives in the
new trip are selected), but we have another corner case:
Just as before, select one or more dives in an expanded dive trip.
Collapse that trip and ctrl-click on another trip. Now you lose the
originally selected dives.

Frankly, if you ctrl-click to add more dives to your selection - just
don't collapse the trips the dives are in?

As this new corner case seems even more artificial than the previous one,
I consider this patch an improvement. But fundamentally I am just battling
all the ways in which gtk's selection handling is messed up. When I get
the selection call back I cannot tell if this is a new selection or an
incremental selection (i.e., a shift-click or ctrl-click).

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