Rewrite gtk dive selection tracking logic

We used to generate a list of possibly changed selections using the gtk
tree selection "selection function".

But that's actually meant to just tell gtk whether an entry can be
selected or not, and our list of possibly changed entries ended up being
stale if the selection change was due to a list entry removal, for
example.

So rip out the old model entirely, and instead just walk the whole
selection that gtk gives us on a selection "change" event.  We throw all
our old selections away when this happens, and just rebuild it all.

This should fix the occasional internal gtklib-quartz assertion that
Henrik is seeing.  And it actually simplifies the code too.

Reported-by: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>