timsort: Add gtk_tim_sort_set_runs()

timsort: Add gtk_tim_sort_set_runs()

... and use it in the SortListModel

Setting runs allows declaring already sorted regions so the sort does
not attempt to sort them again.

This massively speeds up partial inserts where we can reuse the sorted
model as a run and only resort the newly inserted parts.

Benchmarks:

    appending half the model
                    qsort  timsort
    128,000 items    94ms     69ms
    256,000 items   202ms    143ms
    512,000 items   488ms    328ms

    appending 1 item
                    qsort  timsort
      8,000 items   1.5ms    0.0ms
     16,000 items   3.1ms    0.0ms
              ...
    512,000 items     ---    1.8ms