Add option for overwriting playcounts

Fixes #962

src/collection/collectionwatcher.h

@@ -218,6 +218,7 @@ class CollectionWatcher : public QObject {
   bool song_tracking_;
   bool mark_songs_unavailable_;
   int expire_unavailable_songs_days_;
+  bool overwrite_playcount_;
   bool overwrite_rating_;
 
   bool stop_requested_;