More fiddling with the selection

More fiddling with the selection

As expected, this is pretty subtle to get right. But with this change the
code becomes simpler and more straight forward, I think. If the dives in a
group are collapsed, we don't even try to make gtk keep track of their
selection status - we explicitly do so ourselves. This avoids the
artificial expand / collapse around our attempt to force gtk to allow us
to select children that are hidden. But if a dive is expanded, then we
trust gtk to get things right.

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