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/src.pro

@@ -154,7 +154,6 @@ nox {
               autoexpandabledialog.h \
               statsdialog.h \
               messageboxraised.h \
-              statussortfilterproxymodel.h \
               torrentfilterenum.h
 
   SOURCES += mainwindow.cpp \
@@ -179,7 +178,6 @@ nox {
              autoexpandabledialog.cpp \
              statsdialog.cpp \
              messageboxraised.cpp \
-             statussortfilterproxymodel.cpp \
              statusbar.cpp \
              trackerlogin.cpp