[BT] Added TorrentFilesModel::RoleSort for #1742.
[BT] Added sort/filter proxy model, closes #1742.

Also, for #1725.
[BT] Custom files filter model for #1725.

Recursively check if children contain the string in path.
[BT] Exposed search line in UI, closes #1725.
[BT] Show search line only if there is more than one file.
[BT] Added search line clear addon.