libtorrent < 2.0.7 has a bug that add_torrent_alert is posted too early
(before torrent is fully initialized and torrent extensions are created)
so we have to fill initial torrent data in add_torrent_alert handler.
PR #17491.
libtorrent-2.0 introduced a type-safe alternative to void* (lt::client_data_t)
for passing user-data into plugins. qBT doesn't use any user data in its
plugins, but the signatures have changed nevertheless, and qBT need to update to
be compatible with libtorrent-2.0.
"private" subfolders violate the structure of the project,
since the existence of all other subfolders is based on a
different principle.
In addition, there is no clear line between "private" and
"non private".
