Improve "Watched folders" feature

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

src/base/bittorrent/magneturi.cpp

@@ -34,7 +34,6 @@
 #include <libtorrent/sha1_hash.hpp>
 
 #include <QRegularExpression>
-#include <QUrl>
 
 #include "infohash.h"
 
@@ -59,6 +58,8 @@ namespace
 
 using namespace BitTorrent;
 
+const int magnetUriId = qRegisterMetaType<MagnetUri>();
+
 MagnetUri::MagnetUri(const QString &source)
     : m_valid(false)
     , m_url(source)