sortlistmodel: Track item positions

sortlistmodel: Track item positions

The model now tracks the original positions on top of just the items so that
it can remove items in an items-changed emission.

It now takes twice as much memory but removes items much faster.

Benchmarks:

Removing 50% of a model:
                   before    after
   250,000 items    135ms     10ms
   500,000 items    300ms     25ms

Removing 1 item:
     4,000 items    2.2ms      0ms
     8,000 items    4.6ms      0ms
   500,000 items      ---   0.01ms