[BT] AddTorrentFilesModel::RoleSort for #1726.
[BT] Support sorting in file addition dialog, closes #1726.
[BT] TorrentFilesModelBase::IndexForNode().
[BT] Keep parent priorities in sync with children set.
[BT] Show only non-negative priorities.
[BT] Checkable items in torrent files list, closes #1731.