Start up torrents after UI was created. Fixes #6454.

Commit dd0537d changed torrents startup code adding alerts processing
into it. Therefore alerts were processed before UI code subscribed to
signals and therefore part of alerts was not reflected in the UI.

Thus here we do not start torrents in Session constructor, but do that
from Application::exec() after UI was constructed and is ready to process
signals.

src/app/application.cpp

@@ -441,6 +441,9 @@ int Application::exec(const QStringList &params)
         m_paramsQueue.clear();
     }
 
+    // Now UI is ready to process signals from Session
+    BitTorrent::Session::instance()->startUpTorrents();
+
     return BaseApplication::exec();
 }