Make complete type declarations available where needed

src/gui/mainwindow.h

@@ -35,6 +35,8 @@
 #include <QSystemTrayIcon>
 #endif
 
+#include "base/bittorrent/torrent.h"
+
 class QCloseEvent;
 class QFileSystemWatcher;
 class QSplitter;
@@ -57,11 +59,6 @@ class TorrentCreatorDialog;
 class TransferListFiltersWidget;
 class TransferListWidget;
 
-namespace BitTorrent
-{
-    class Torrent;
-}
-
 namespace Net
 {
     struct DownloadResult;