Some refactoring. Replaced all "listeners" list by instances of

CopyOnWriteArrayList, to allow listeners to unregister themselves due to
an event while preventing ConcurrentModificationExceptions.
Modified all GameDataElement.elementChanged concrete implementation to
remove the backlink from the oldOne.An element pointed by an altered
element will not show the game source element in its backlink list
anymore.

src/com/gpl/rpg/atcontentstudio/ui/WorkspaceActions.java

@@ -12,6 +12,8 @@ import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import java.util.concurrent.ConcurrentSkipListSet;
+import java.util.concurrent.CopyOnWriteArrayList;
 
 import javax.swing.Action;
 import javax.swing.JFileChooser;
@@ -452,10 +454,8 @@ public class WorkspaceActions {
 		public void putValue(String key, Object value) {
 			PropertyChangeEvent event = new PropertyChangeEvent(this, key, values.get(key), value);
 			values.put(key,  value);
-			synchronized(listeners) {
-				for (PropertyChangeListener l : listeners) {
-					l.propertyChange(event);
-				}
+			for (PropertyChangeListener l : listeners) {
+				l.propertyChange(event);
 			}
 		}
 
@@ -463,10 +463,8 @@ public class WorkspaceActions {
 		public void setEnabled(boolean b) {
 			PropertyChangeEvent event = new PropertyChangeEvent(this, "enabled", isEnabled(), b);
 			enabled = b;
-			synchronized(listeners) {
-				for (PropertyChangeListener l : listeners) {
-					l.propertyChange(event);
-				}
+			for (PropertyChangeListener l : listeners) {
+				l.propertyChange(event);
 			}
 		}
 
@@ -475,20 +473,16 @@ public class WorkspaceActions {
 			return enabled;
 		}
 
-		private Set<PropertyChangeListener> listeners = new HashSet<PropertyChangeListener>();
+		private List<PropertyChangeListener> listeners = new CopyOnWriteArrayList<PropertyChangeListener>();
 		
 		@Override
 		public void addPropertyChangeListener(PropertyChangeListener listener) {
-			synchronized(listeners) {
-				listeners.add(listener);
-			}
+			listeners.add(listener);
 		}
 
 		@Override
 		public void removePropertyChangeListener(PropertyChangeListener listener) {
-			synchronized(listeners) {
-				listeners.remove(listener);
-			}
+			listeners.remove(listener);
 		}
 		
 	}