After resorting, we now re-map every selected index so it matches the
new row mappings. This makes the process table view in SystemMonitor
behave normally again :^)
This breaks GSortingProxyModel selection preservation across resorts.
I'm not yet sure how we're going to solve that, but it's going to have
to work a bit differently than before, since the model itself no longer
knows what's selected.
Selection is now managed by GModelSelection which allows us to select
any arbitrary number of items, and to have different selections in
different views onto the same model. Pretty sweet. :^)
