Add a lint test for audio notifications.

Only traits are linted - the UI still hardcodes
too many audio references for this to be worthwhile.

OpenRA.Mods.Common/Traits/World/StartGameNotification.cs

@@ -16,6 +16,7 @@ namespace OpenRA.Mods.Common.Traits
 {
 	class StartGameNotificationInfo : ITraitInfo
 	{
+		[NotificationReference("Speech")]
 		public readonly string Notification = "StartGame";
 
 		public object Create(ActorInitializer init) { return new StartGameNotification(this); }