avoid deadlock when buoy listeners for cache invalidation for first

leg's direction are registered

java/com.sap.sailing.domain/src/com/sap/sailing/domain/tracking/GPSFixTrack.java

@@ -25,6 +25,8 @@ public interface GPSFixTrack<ItemType, FixType extends GPSFix> extends Track<Fix
      */
     void addListener(GPSTrackListener<ItemType> listener);
     
+    void removeListener(GPSTrackListener<ItemType> listener);
+    
     ItemType getTrackedItem();
 
     /**

java/com.sap.sailing.domain/src/com/sap/sailing/domain/tracking/impl/GPSFixTrackImpl.java

@@ -53,6 +53,13 @@ public class GPSFixTrackImpl<ItemType, FixType extends GPSFix> extends TrackImpl
         }
     }
     
+    @Override
+    public void removeListener(GPSTrackListener<ItemType> listener) {
+        synchronized (listeners) {
+            listeners.remove(listener);
+        }
+    }
+    
     /**
      * To iterate over the resulting listener list, synchronize on the iterable returned. Only this will avoid
      * {@link ConcurrentModificationException}s because listeners may be added on the fly, and this object will