Swap in vohof's Transmission back-end. Thinner wrapper, more features.

Old library not yet removed, while I'm doing the transition
Implement CopyTorrentFiles::execute() and ::undo()
Implement DeleteEmptyDirs::execute() and ::undo(), no tests
Implement DeleteTorrent::execute() and DestroyTorrent::execute(). No tests.
Implement DeleteAllTorrents (tests) and DestroyAllTorrents (no tests)
Implement StartTorrent and StartAllTorrents. No tests.
Implement StopTorrent and StopAllTorrents. No tests.
Implement QueueTorrent and QueueAllTorrents. No tests.
placeholders