sortlistmodel: Properly compute runs

sortlistmodel: Properly compute runs

When updating a (partially) sorted model, take the known runs for the
existing sort and apply them to the new sort. That way, we don't have to
check the whole model again.

Benchmarks:

      appending half the items to a model of strings
                        old      new
      512,000 items   437ms    389ms
    1,024,000 items  1006ms    914ms

      appending 10% of the items to a model of strings
                        old      new
      512,000 items   206ms    132ms
    1,024,000 items   438ms    301ms

      appending 1 item to a model of strings
                        old      new
       64,000 items   1.8ms   0.00ms
      512,000 items     ---   0.01ms