Improve "Watched folders" feature

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

src/base/bittorrent/session.cpp

@@ -338,6 +338,8 @@ namespace
 #endif
 }
 
+const int addTorrentParamsId = qRegisterMetaType<AddTorrentParams>();
+
 // Session
 
 Session *Session::m_instance = nullptr;