Improve "Watched folders" feature

Make "file system watcher" an application core component
and separate it from its presentation model.

src/base/bittorrent/torrentinfo.cpp

@@ -75,6 +75,8 @@ namespace
     }
 }
 
+const int torrentInfoId = qRegisterMetaType<TorrentInfo>();
+
 TorrentInfo::TorrentInfo(std::shared_ptr<const lt::torrent_info> nativeInfo)
 {
     m_nativeInfo = std::const_pointer_cast<lt::torrent_info>(nativeInfo);