Add event.has and default BackgroundWorker send* events to null messages

lime/app/Event.hx

@@ -74,6 +74,14 @@ class Event<T> {
 	}
 	
 	
+	public function has (listener:T):Bool {
+		
+		var index = listeners.indexOf (listener);
+		return (index > -1);
+		
+	}
+	
+	
 	public function remove (listener:T):Void {
 		
 		var index = listeners.indexOf (listener);

lime/system/BackgroundWorker.hx

@@ -74,7 +74,7 @@ class BackgroundWorker {
 	}
 	
 	
-	public function sendComplete (message:Dynamic):Void {
+	public function sendComplete (message:Dynamic = null):Void {
 		
 		#if (cpp || neko)
 		
@@ -95,7 +95,7 @@ class BackgroundWorker {
 	}
 	
 	
-	public function sendError (message:Dynamic):Void {
+	public function sendError (message:Dynamic = null):Void {
 		
 		#if (cpp || neko)
 		
@@ -116,7 +116,7 @@ class BackgroundWorker {
 	}
 	
 	
-	public function sendProgress (message:Dynamic):Void {
+	public function sendProgress (message:Dynamic = null):Void {
 		
 		#if (cpp || neko)