Avoid changing selection status when collapsing/expanding groups

This tries to avoid the problem mentioned in commit972669d6363c ("Rework
dive selection logic"), where a selection of dives hidden by collapsing
the group gets forgotten about by gtk.  It does so by always marking the
group selected when it is collapsed with any selected children.

We also avoid selecting new children when a group is selected that
already has at least *some* children selected already.  This way we do
minimal damage to existing selections when working with dive group
selections.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>