[#2315] [GTKUI] Potential fix for lost window on Win32
Prevent private flagged torrents auto-merging trackers

When adding a torrent already in session any new trackers are merged
to the exiting torrent but this is an unwanted feature for private
flagged torrents.
[#2418] Fix WebUI error when adding non-ascii torrent

json.dumps attempts to decode (utf8) the 'path' entry which had a
alternative encoding. The solution is to ensure the 'path' entry is
utf8 encoded and remove the unneeded 'path.utf-8' entry.

As self.__m_metadata["info"]["files"] is updated the later code
checking and decoding the 'path' entry can be removed.