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/model/maps/TMXMapSet.java

@@ -108,7 +108,6 @@ public class TMXMapSet implements ProjectTreeNode {
 								for (WatchEvent<?> event : wk.pollEvents()) {
 									Path changed = (Path) event.context();
 									String name = changed.getFileName().toString();
-									System.out.println("Changed: "+name);
 									String id = name.substring(0, name.length() - 4);
 									TMXMap map = getMap(id);
 									if (map != null) {