Show input box to filter torrent list only when the list is shown

It's misleading to show the filter on every tab when it works only
for the torrent list.

src/mainwindow.h

@@ -184,6 +184,7 @@ private:
   bool force_exit;
   bool ui_locked;
   LineEdit *search_filter;
+  QAction *searchFilterAct;
   // Keyboard shortcuts
   QShortcut *switchSearchShortcut;
   QShortcut *switchSearchShortcut2;