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/Power/Player/PowerManager.cs

@@ -20,6 +20,8 @@ namespace OpenRA.Mods.Common.Traits
 	public class PowerManagerInfo : ITraitInfo, Requires<DeveloperModeInfo>
 	{
 		public readonly int AdviceInterval = 250;
+
+		[NotificationReference("Speech")]
 		public readonly string SpeechNotification = null;
 
 		public object Create(ActorInitializer init) { return new PowerManager(init.Self, this); }