sortlistmodel: Make key generation part of the step function

sortlistmodel: Make key generation part of the step function

SSave the missing keys as a bitset and iterate over that bitset in the
step function.

Solves the problem with a large UI block at the beginning of a sort
operation when all the keys were generated, in particular when key
generation was slow.

Benchmarks for maximum time taken by a single main loop callback:

     initial sort with complex GFileInfo keys
                       old      new
      32,000 items   137ms      3ms
     128,000 items   520ms     31ms

     initial sort with string keys
                       old      new
      32,000 items   187ms      1ms
     128,000 items   804ms      3ms