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/Sound/ActorLostNotification.cs

@@ -15,7 +15,9 @@ namespace OpenRA.Mods.Common.Traits.Sound
 {
 	class ActorLostNotificationInfo : ITraitInfo
 	{
+		[NotificationReference("Speech")]
 		public readonly string Notification = "UnitLost";
+
 		public readonly bool NotifyAll = false;
 
 		public object Create(ActorInitializer init) { return new ActorLostNotification(this); }

OpenRA.Mods.Common/Traits/Sound/AnnounceOnSeen.cs

@@ -21,6 +21,7 @@ namespace OpenRA.Mods.Common.Traits.Sound
 		[Desc("Should there be a radar ping on enemies' radar at the actor's location when they see him")]
 		public readonly bool PingRadar = false;
 
+		[NotificationReference("Speech")]
 		public readonly string Notification = null;
 
 		public readonly bool AnnounceNeutrals = false;

OpenRA.Mods.Common/Traits/Sound/CaptureNotification.cs

@@ -15,12 +15,14 @@ namespace OpenRA.Mods.Common.Traits.Sound
 {
 	public class CaptureNotificationInfo : ITraitInfo
 	{
+		[NotificationReference("Speech")]
 		[Desc("The speech notification to play to the new owner.")]
 		public readonly string Notification = "BuildingCaptured";
 
 		[Desc("Specifies if Notification is played with the voice of the new owners faction.")]
 		public readonly bool NewOwnerVoice = true;
 
+		[NotificationReference("Speech")]
 		[Desc("The speech notification to play to the old owner.")]
 		public readonly string LoseNotification = null;