Merge StatusSortFilterProxyModel into TransferListSortModel

I thought merging all 3 sort-filter proxy models into one should
speedup dataChanged() signal. As turned out this is not the case. The
time difference is within inaccuracy range, so this doesn't affect
performance.

But I still think it is good to merge them into one proxy model as it
simplifies code.

src/transferlistwidget.h

@@ -40,7 +40,6 @@ class MainWindow;
 class TransferListDelegate;
 class TransferListSortModel;
 class TorrentModel;
-class StatusSortFilterProxyModel;
 
 QT_BEGIN_NAMESPACE
 class QShortcut;
@@ -114,7 +113,6 @@ private:
   TransferListDelegate *listDelegate;
   TorrentModel *listModel;
   TransferListSortModel *nameFilterModel;
-  StatusSortFilterProxyModel *statusFilterModel;
   QSortFilterProxyModel *labelFilterModel;
   QBtSession* BTSession;
   MainWindow *main_window;