sortlistmodel: Make the sort callback useful

sortlistmodel: Make the sort callback useful

1. Run step() for a while to avoid very short steps
   This way, we batch items-changed() emissions.

2. Track the change region accurately
   This way, we can avoid invalidating the whole list if our step just
   touched a small part of a huge list.
   As this is a merge sort, this is a common occurence when we're buys
   merging chunks: The rest of the model outside those chunks isn't
   changed.

Note that the tracking is accurate: It determines the minimum change
region in the model.

This will be important, because the testsuite is going to test this.