v0.56.0

Changed

• A migration to a network share copies four files at once. One file
  at a time left a fast link and the NAS mostly idle. Every copy is still
  read back and compared with its original before it counts, and catalog
  entries are still repointed one by one, in order. A file whose place in
  the target is already taken, or wanted by another file, is handled one at
  a time as before. Local drives still copy one file at a time.

Fixed

• After Start over, the emptied target stayed listed as a source, so
  planning into the same place again was refused ("the target overlaps the
  source") until it was removed by hand in Settings.