Add index and filter combo box to external help window

Change-Id: Icc815379220c55d9d7dc5d154278ba21ce2e9984
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/help/localhelpmanager.h

@@ -73,6 +73,7 @@ public:
 
     static QAbstractItemModel *filterModel();
     static void setFilterIndex(int index);
+    static int filterIndex();
 
     static void updateFilterModel();