Rework dive selection logic

Rework dive selection logic

This completely changes how we keep track of selected dives: instead of
having an array listing the selection ("selectiontracker") or trusting
the gtk selection information, just save the information about whether a
dive is selected in the dive itself.

That makes it trivial to keep track of the state of selection across
group collapse/expand events, or when changing the tree view model.  It
also ends up simplifying the code and logic in other ways.

HOWEVER, it does currently (re-)introduce an annoying oddity with gtk:
if you collapse a dive trip that has individual selections, gtk will
forget those selections ("out of sight, out of mind"), and when you do
*new* selections, the old hidden ones remain.

So there's some games required to make gtk do sane things.  We may need
to either explicitly drop selections when collapsing trips, or make sure
the group entry gets selected when collapsing a group that has
selections in it. Or something.

There may be other issues introduced by this too.

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